Testing if a variable is in an expression

The has command determines whether or not an expression contains a given variable.

has takes two arguments:
- expr, an expression.
- x, the name of a variable.
has(expr,x) returns 0 if expr does not contain x and the 1-based index of x in lname(expr) otherwise.

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

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