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

### 13.16.1 Checking if a point is on an object in the plane: is_element

See Section 14.13.1 for checking elements in three-dimensional geometry.

The is_element command determines whether or not a point is on a geometric object.

- is_element takes two arguments:
  - P, a point.
  - G, a geometric object.
- is_element(P,G) returns 1 if P is an element of G and returns 0 otherwise.

Examples.

- Input:
  is_element(-1-i, line(0,1+i))
  Output:

  1

- Input:
  is_element(i, line(0,1+i))
  Output:

  0