Keep branch-local names local

Assignments to an existing mutable binding survive a branch. New names declared inside a branch belong to that branch:

def main() -> Result[(), IoError | OverflowError[i64]]:
    mut price = 100
    discounted = True
    if discounted:
        discount = 20
        price = price - discount
    print(price)?
    Ok(())

80

Declare a mutable binding before the branch when subsequent code needs the updated value. For a new selected value, bind an inline conditional when each branch is one expression. A block conditional also supports branch-local work:

def main() -> Result[(), IoError | OverflowError[i64]]:
    discounted = True
    price = if discounted:
        discount = 20
        100 - discount
    else:
        100
    print(price)?
    Ok(())

80

Only price survives the block; discount still belongs to its branch. This keeps price immutable when it is selected once. The block must be the whole right-hand side; see conditional values.