```
float predicate_incircle(vector2 a, vector2 b, vector2 c, vector2 d)
```

Given 3 points `a`

, `b`

, and `c`

on the plane, return a positive value if `d`

is inside
the circumcircle of the triangle `abc`

, a negative value if `d`

is outside, and zero
if `d`

is exactly on the circumcircle.

More precisely, this function computes the determinant of the matrix:

[a_x a_y a^2 1; b_x b_y b^2 1; c_x c_y c^2 1; d_x d_y d^2 1]

…with a guaranteed
correct sign, where `a^2`

, `b^2`

, `c^2`

, and `d^2`

are the squared lengths of the
respective input vectors.

intersect |