IfStatement

Corresponds directly to an ``if'' statement in C.

    concrete IfStatement : Statement
      {
        Expression * owner condition in source_ops;
        Statement * owner then_part in child_statements;
        Statement * owner else_part in child_statements build {0};
      };

condition	First the condition operand is evaluated. The type of the condition must be a BooleanType.
then_part	The then_part is executed if the condition evaluates to true. NULL is a valid value for this field.
else_part	The else_part is executed if the condition evaluates to false. NULL is a valid value for this field.