stmtlist: StmtList(expr,expr);
expr : linearST
     | const
     | Sub(expr,expr)
     | Abs(expr)
     | Assign(Var,expr);
linearST : s | t
	 | const
	 | Mul(const,linearST)
	 | Sub(linearST,linearST)
	 | Assign(Var,linearST);
const : ConstFloat
      | Mul(const,const);