It seems there is a way to exit the program when a check for the existence condition of the tetrahedron is "NO". We put the program text in a pseudo-loop where the break statement works. If the check passes, then the print ("GO") is executed; if not, the print ("GO") is not executed. A simple check by point F: if its third coordinate is other than 0, then the program is executed, for example, point (F, 1, 1, 0.000001), if its third coordinate is 0, then the program is not executed.
for j to 1 do
point(A, 0, 0, 0), point(B, 2, 2, 0), point(C, 0, 2, 0), point(F, 1, 1, 0):
gtetrahedron(T1, [A, B, C, F]):
i := nops(op(0, detail(T1))); if i = 1 then print("Ok, gtetrahedron") else print("No gtetrahedron, NO"); break end if: