change parser for def

>>> def add(num1,num2) then return num1+num2 end x=1 y=2

( walley_statements
    ( statements
        ( =(id add)
            ( func_value
                ( def)( params(id num1)(id num2))
                ( statements( return)( +(id num1)(id num2)))
                ( statements( end))
            )
        )
    )
    ( statements( =(id x)(num 1)))
    ( statements( =(id y)(num 2)))
)