* The pointed to AVExpr must be freed with av_expr_free() by the user
* when it is not needed anymore.
* @param s expression as a zero terminated string, for example "1+2^3+5*5+sin(2/3)"
* The pointed to AVExpr must be freed with av_expr_free() by the user
* when it is not needed anymore.
* @param s expression as a zero terminated string, for example "1+2^3+5*5+sin(2/3)"