def p_constraint_list(self, p):
    ' constraint_list : constraint_list COMMA constraint\n                            | constraint '
    if (len(p) == 2):
        p[0] = [p[1]]
    else:
        p[0] = p[1]
        p[1].append(p[3])
constraint_list : constraint_list COMMA constraint
| constraint
def p_constraint_list(self, p):
    ' constraint_list : constraint_list COMMA constraint\n                            | constraint '
    if (len(p) == 2):
        p[0] = [p[1]]
    else:
        p[0] = p[1]
        p[1].append(p[3])