This section of the tutorial builds up on the previous one, and will reuse the same running example, but all the Catala code necessary to execute the example is included below for reference.

```catala
declaration structure Individual:
  data income content money
  data number_of_children content integer

declaration scope IncomeTaxComputation:
  input current_date content date
  input individual content Individual
  input overseas_territories content boolean
  internal tax_rate content decimal
  output income_tax content money
```

## Article 1

 The income tax for an individual is defined as a fixed percentage of the
 individual's income over a year.

```catala
scope IncomeTaxComputation:
  definition income_tax equals
    individual.income * tax_rate
```

## Article 2 (old version before 2000)

The fixed percentage mentioned at article 1 is equal to 20 %.

```catala
scope IncomeTaxComputation:
  label article_2
  definition tax_rate under condition
    current_date < |2000-01-01|
  consequence equals 20%
```

## Article 2 (new version after 2000)

The fixed percentage mentioned at article 1 is equal to 21 % %.

```catala
scope IncomeTaxComputation:
  # Simply use the same label "article_2" as the previous definition to group
  # them together
  label article_2
  definition tax_rate under condition
    current_date >= |2000-01-01|
  consequence equals 21%
```

## Article 3

If the individual is in charge of 2 or more children, then the fixed
percentage mentioned at article 1 is equal to 15 %.

```catala
scope IncomeTaxComputation:
  label article_3 exception article_2
  definition tax_rate under condition
    individual.number_of_children >= 2
  consequence equals 15%
```

## Article 4

Individuals earning less than $10,000 are exempted of the income tax mentioned
at article 1.

```catala
scope IncomeTaxComputation:
  label article_4 exception article_3
  definition tax_rate under condition
    individual.income <= $10,000
  consequence equals 0%
```

## Article 5

Individuals earning more than $100,000 are subjects to a tax rate of
30%, regardless of their number of children.

```catala
scope IncomeTaxComputation:
  label article_5 exception article_3
  definition tax_rate under condition
    individual.income > $100,000
  consequence equals 30%
```

## Article 6

In the overseas territories, the tax rate for individuals earning
more than $100,000 specified at article 5 is reduced to 25 %.

```catala
scope IncomeTaxComputation:
  label article_6 exception article_5
  definition tax_rate under condition
    individual.income > $100,000 and overseas_territories
  consequence equals 25%
```

## Article 7

When several individuals live together, they are collectively subject to
the household tax. The household tax owed is $10000 per individual of the household,
and half the amount per children.

```catala
declaration scope HouseholdTaxComputation:
  input individuals content list of Individual

  output household_tax content money

declaration scope HouseholdTaxIndividualComputation:
  input individual content Individual
  input overseas_territories content boolean
  input current_date content date

  income_tax_computation scope IncomeTaxComputation

  output household_tax content money
```

```catala
# scope HouseholdTaxIndividualComputation:
#   # This definition is disabled because of the new one in Article 8.
#   definition household_tax equals
#     $10000 * (1.0 + decimal of individual.number_of_children / 2.0)

# The definition of household tax for the whole household below is wrong and
# will be refactored in the next section.
scope HouseholdTaxComputation:
  definition household_tax equals
    let number_of_individuals equals number of individuals in
    let number_of_children equals
      sum integer
        of individual.number_of_children for individual among individuals
    in
    $10,000
    * (
      decimal of number_of_individuals
      + decimal of number_of_children / 2.0
    )
```

## Article 8

The amount of income tax paid by each individual can be deducted from the
share of household tax owed by this individual.

```catala
scope HouseholdTaxIndividualComputation:
  definition income_tax_computation.individual equals
    individual
  definition income_tax_computation.overseas_territories equals
    overseas_territories
  definition income_tax_computation.current_date equals
    current_date

  definition household_tax equals
    let tax equals
      $10,000 * (1.0 + decimal of individual.number_of_children / 2.0)
    in
    let deduction equals income_tax_computation.income_tax in
    # Don't forget to cap the deduction!
    if deduction > tax then $0 else tax - deduction
```

## Test

```catala
declaration scope Test:
  output computation content IncomeTaxComputation

scope Test:
  definition computation equals
    output of IncomeTaxComputation with {
      -- individual:
        Individual {
          -- income: $20,000
          -- number_of_children: 0
        }
      -- overseas_territories: false
      -- current_date: |1999-01-01|
    }

declaration scope TestHousehold:
  output computation content HouseholdTaxComputation

scope TestHousehold:
  definition computation equals
    output of HouseholdTaxComputation with {
      -- individuals:
        [ Individual {
            -- income: $15,000
            -- number_of_children: 0
          } ;
          Individual {
            -- income: $80,000
            -- number_of_children: 2
          } ]
    }

declaration scope TestIndividualHousehold:
  output computation content HouseholdTaxIndividualComputation

scope TestIndividualHousehold:
  definition computation equals
    output of HouseholdTaxIndividualComputation with {
      -- individual:
        Individual {
          -- income: $15,000
          -- number_of_children: 0
        }
      -- current_date: |1999-01-01|
      -- overseas_territories: false
    }
```

For reference, here is the final version of the Catala code consolidated at the end of this section of the tutorial.

```catala
declaration structure Individual:
  data income content money
  data number_of_children content integer

declaration scope IncomeTaxComputation:
  input current_date content date
  input individual content Individual
  input overseas_territories content boolean
  internal tax_rate content decimal
  output income_tax content money
```

## Article 1

 The income tax for an individual is defined as a fixed percentage of the
 individual's income over a year.

```catala
scope IncomeTaxComputation:
  definition income_tax equals
    individual.income * tax_rate
```

## Article 2 (old version before 2000)

The fixed percentage mentioned at article 1 is equal to 20 %.

```catala
scope IncomeTaxComputation:
  label article_2
  definition tax_rate under condition
    current_date < |2000-01-01|
  consequence equals 20%
```

## Article 2 (new version after 2000)

The fixed percentage mentioned at article 1 is equal to 21 % %.

```catala
scope IncomeTaxComputation:
  # Simply use the same label "article_2" as the previous definition to group
  # them together
  label article_2
  definition tax_rate under condition
    current_date >= |2000-01-01|
  consequence equals 21%
```

## Article 3

If the individual is in charge of 2 or more children, then the fixed
percentage mentioned at article 1 is equal to 15 %.

```catala
scope IncomeTaxComputation:
  label article_3 exception article_2
  definition tax_rate under condition
    individual.number_of_children >= 2
  consequence equals 15%
```

## Article 4

Individuals earning less than $10,000 are exempted of the income tax mentioned
at article 1.

```catala
scope IncomeTaxComputation:
  label article_4 exception article_3
  definition tax_rate under condition
    individual.income <= $10,000
  consequence equals 0%
```

## Article 5

Individuals earning more than $100,000 are subjects to a tax rate of
30%, regardless of their number of children.

```catala
scope IncomeTaxComputation:
  label article_5 exception article_3
  definition tax_rate under condition
    individual.income > $100,000
  consequence equals 30%
```

## Article 6

In the overseas territories, the tax rate for individuals earning
more than $100,000 specified at article 5 is reduced to 25 %.

```catala
scope IncomeTaxComputation:
  label article_6 exception article_5
  definition tax_rate under condition
    individual.income > $100,000 and overseas_territories
  consequence equals 25%
```

## Article 7

When several individuals live together, they are collectively subject to
the household tax. The household tax owed is $10000 per individual of the household,
and half the amount per children.

```catala
declaration scope HouseholdTaxComputation:
  input individuals content list of Individual
  input overseas_territories content boolean
  input current_date content date

  internal shares_of_household_tax
    content list of HouseholdTaxIndividualComputation
  internal total_deduction content money
    state base
    state capped

  output household_tax content money
    state base
    state deduction

declaration scope HouseholdTaxIndividualComputation:
  input individual content Individual
  input overseas_territories content boolean
  input current_date content date

  income_tax_computation scope IncomeTaxComputation

  output household_tax content money
  output deduction content money
```

```catala
scope HouseholdTaxIndividualComputation:
  definition household_tax equals
    $10000 * (1.0 + decimal of individual.number_of_children / 2.0)

scope HouseholdTaxComputation:
  definition shares_of_household_tax equals
    (
      output of HouseholdTaxIndividualComputation with {
        -- individual: individual
        -- overseas_territories: overseas_territories
        -- current_date: current_date
      }
    )
      for individual among individuals

  definition household_tax
    state base
  equals
    sum money
      of share_of_household_tax.household_tax
      for share_of_household_tax among shares_of_household_tax
```

## Article 8

The amount of income tax paid by each individual can be deducted from the
share of household tax owed by this individual.

```catala
scope HouseholdTaxIndividualComputation:
  definition income_tax_computation.individual equals
    individual
  definition income_tax_computation.overseas_territories equals
    overseas_territories
  definition income_tax_computation.current_date equals
    current_date

  definition deduction equals
    if income_tax_computation.income_tax > household_tax then household_tax
    else
      income_tax_computation.income_tax

scope HouseholdTaxComputation:
  definition total_deduction
    state base
  equals
    sum money
      of share_of_household_tax.deduction
      for share_of_household_tax among shares_of_household_tax

  definition household_tax
    state deduction
  equals
    if total_deduction > household_tax then $0
    else household_tax - total_deduction
```

## Article 9

The deduction granted at article 8 is capped at $8,500 for the whole household.

```catala
scope HouseholdTaxComputation:
  definition total_deduction
    state capped
  equals
    if total_deduction > $8,500 then $8,500 else total_deduction
```

## Test

```catala
declaration scope Test:
  output computation content IncomeTaxComputation

scope Test:
  definition computation equals
    output of IncomeTaxComputation with {
      -- individual:
        Individual {
          -- income: $20,000
          -- number_of_children: 0
        }
      -- overseas_territories: false
      -- current_date: |1999-01-01|
    }

declaration scope TestHousehold:
  output computation content HouseholdTaxComputation

scope TestHousehold:
  definition computation equals
    output of HouseholdTaxComputation with {
      -- individuals:
        [ Individual {
            -- income: $15,000
            -- number_of_children: 0
          } ;
          Individual {
            -- income: $80,000
            -- number_of_children: 2
          } ]
      -- overseas_territories: false
      -- current_date: |1999-01-01|
    }
```