ug二次开发中创建表达式

急用请各位高手帮帮忙！！！小女子感激不尽！！！
我建模编程时需要用到表达式，请问各位高手：
怎样编写创建表达式的代码啊？
我在创建表达式时用到ug中自带的变量t，请问各位：
怎样使用这个自变量t啊？？

extern int UF_MODL_create_exp (
char * expr_str );

char *  expr_str Input New expression. The maximum number of
characters for an expression name (lhs or left hand
side) is 130 characters. The maximum number for
the entire expression is 132 characters. This
includes all three pieces: lhs, equal sign, and rhs.

Environment
Internal and External