(p)
parameter : ID
                |  ID "=" expression
parameter : ID
                |  ID "=" expression
parameter : ID | ID = expression
def p_parameter(p):
    '''parameter : ID
                |  ID "=" expression'''
    p[0] = ScadParameter(p[1], len(p) == 4)
def p_parameter ( p ) : p [ 0 ] = ScadParameter ( p [ 1 ] , len ( p ) == 4 )