[math-fun] A function you've probably written more than once

Interseg[z1_,z2_,z3_,z4_]:=Block[{z12=z1-z2,z34=z3-z4}, (z34*Im[z2*z1\[Conjugate]]+z12*Im[z3*z4\[Conjugate]])/Im[z34*z12\[Conjugate]]] the intersection of the two lines determined by {z1,z2} and {z3,z4}. (This can't be new. But a little Googling found me nothing this short.) --rwg