### 2.54.5 Test if a variable is in an expression : `has`

`has` takes as argument an expression and the name of a
variable.

`has` returns `1` if this variable is in this expression, and else
returns `0`.

Input :

`has(x*y*sin(x),y)`

Output :

`1`

Input :

`has(x*y*sin(x),z)`

Output :

`0`