'expression  : assignment_expression
| expression COMMA assignment_expression'

def p_expression(self, p):

    if (len(p) == 2):
        p[0] = p[1]
    else:
        if (not isinstance(p[1], c_ast.ExprList)):
            p[1] = c_ast.ExprList([p[1]], p[1].coord)
        p[1].exprs.append(p[3])
        p[0] = p[1]
