Unified disposal for two-way loop in design of GiNaC compiler

A new method to dispose the two-way loop in syntax analysis was introduced.In this method,both increased and decreased loops were treated as one pattern.The recognition and distinction between them would be dealt with until the object codes was generated.Using this method,the weight of each part in compiler will be more equitable.At the same time,this method provides more information for code optimizing.GiNaC is an open-source symbolic computing package based on Linux,and the method implemented in GiNaC compiler achieves a satisfactory result.