Add some comments.

source/app.d

@@ -11,9 +11,12 @@ const double internalInjuries = 25.0;
 const double fatal = 40.0;
 const double puree = 100.0;
 
-const Acceleration g = 9.8 * metre / second /second; // about 35.3 km/h/s
+const Acceleration g = 9.8 * metre / second / second;  // about 35.3 km/h/s
 const Mass typicalHuman = 80 * kilogram;
 
+/++
+ + Prompt the user for a plausible number from stdin.
+ +/
 double getVelocity() {
     string v = "";
     while (!isNumeric(v)) {
@@ -24,6 +27,7 @@ double getVelocity() {
 }
 
 void main(string[] argv) {
+    // Fetch a number from a cli argument, or prompt for one from stdin.
     double velocity = 0;
     auto args = getopt(argv,
         "velocity|v", "How fast The Flash was going", &velocity
@@ -43,6 +47,7 @@ void main(string[] argv) {
     Force f = typicalHuman * a;
     double accel_g = a / g;
 
+    // Output some opinionated results.
     writefln("The Flash goes from stand-still to %s in a tenth of a second, thus accelerating at %.1f g.", "%.0f km/h".siFormat(v), accel_g);
     writefln("This would exert a force of %s on a typical human body.", "%.1f kN".siFormat(f));