Checking whether four points form a harmonic division

The is_harmonic command determines whether or not four points form a harmonic division.

is_harmonic takes four arguments: P,Q,R,S, four points.
is_harmonic(P,Q,R,S) returns 1 if P,Q,R and S form a harmonic range and 0 otherwise.

is_harmonic(0,2,3/2,3)

is_harmonic(0,1+i,1,i)