Conditionally executes code.
Used where code needs to be executed only if some condition is present.
expression shall be an expression, convertible to bool.
If it evaluates to true, control is passed to statement_true, statement_false (if present) is not executed.
Otherwise, control is passed to statement_false, statement_true is not executed.
The following example demonstrates several usage cases of the
first is false i == 3 i != 3 is false