(p)
field_type : ref_type
                  | definition_type
field_type : ref_type
                  | definition_type
field_type : ref_type | definition_type
def p_field_type(p):
    '''field_type : ref_type
                  | definition_type'''
    p[0] = p[1]
def p_field_type ( p ) : p [ 0 ] = p [ 1 ]