Check if a point is on an object in the plane: is_element

### 12.16.1 Check if a point is on an object in the plane: is_element

See section 13.13.1 for checking elements in three-dimensional geometry.

The is_element command takes two arguments, a point and a geometric object.

is_element returns 1 if the point is an element of the object and returns 0 otherwise.
Input:

is_element(-1-i, line(0,1+i))

Output:

1

Input:

is_element(i, line(0,1+i))

Output:

0