void myActions() { // actions to be executed at each frame
    scribe("Jarek Rossignac's edge / edge intersection (general position)");         // prints text at top of the graphic window (see TAB UI), line 0
    if ((mousePressed)&&(!keyPressed)) dragPoint();    // while mouse pressed moves the selected point by mouse displacement (in TAB polygon)
    fill(black);              // set fill color to black (for writing text in the windows)
    strokeWeight(3);          // set stroke width to 3
    stroke(blue);             // set current color to blue
    pt A=P[0]; A.show(2); A.showLabel("A");  // Assign variable point A to P[0], show it as a disk of radius 4, write its label "A" next to it
    pt B=P[1]; B.show(2); B.showLabel("B"); 
    A.to(B);                                // draw a line from A to B in current color (see TAB geo2D, end of class pt)
    stroke(green);
    pt C=P[2]; C.show(2); C.showLabel("C");
    pt D=P[3]; D.show(2); D.showLabel("D");
    C.to(D);
    stroke(orange);
    showEdgeEdgeIntersection(A,B,C,D);      // WHERE THE ACTUAL INTERSECTION IS COMPUTED AND DISPLAYED (see below)
    fill(black);     
    };

void showEdgeEdgeIntersection(pt A, pt B, pt C, pt D) { // show intersection point between two edges if it exists
  if (edgesCross(A, B, C, D)) {        // test if edge(A,B) and edge(C,D) actually intersect (see below)
    vec T=V(A,B);                      // make vector T=AB
    vec N = R(V(C,D));                 // makes vector N by rotating CD by 90 degrees 
    vec AC = V(A,C);
    float t = dot(AC,N) / dot(T,N);    // compute intersection parameter
    show(S(A,t,T));                    // show() shows a point as a small disk. S() computes a point on a line (see TAB geo2D)
                                                           // EXAMPLES OF DEBUGGING TECHNIQUES
               if (debugging) println("t="+t);             // prints current value of t in the text output pane only once each time user presses '?'
               scribe("t="+t,1);                           // prints the value of t at the top of the graphics window (line 1) at each frame
               stroke(cyan,200); arrow(M(C,D),50,U(N));    // shows semi-transparent 50 pixel arrow from (C+D)/2 aligned with N 
    }
  }
  
boolean ordered(pt A, pt B, pt C) {return (0<dot(V(A,B),V(A,C))) && (dot(V(A,B),V(A,C))<dot(V(A,C),V(A,C))); }  // tests whether B falls between A and C
boolean edgesCross(pt A, pt B, pt C, pt D) {return (dot(U(R(V(A,B))),V(A,C))>0 != dot(U(R(V(A,B))),V(A,D))>0) // tests whether two edges cross
                                                && (dot(U(R(V(C,D))),V(C,A))>0 != dot(U(R(V(C,D))),V(C,B))>0) ; }