function preload() {
  // load SVG modules
  modules = [];

  // METHOD 1: Looping through local files is efficient
  // for (var i = 0; i < 16; i++) {
  //   modules[i] = loadImage('data/' + nf(i, 2) + '.svg');
  // }

  // METHOD 2: Read files one-by-one
  modules[0] = loadImage('data/00.svg');
  modules[1] = loadImage('data/01.svg');
  modules[2] = loadImage('data/02.svg');
  modules[3] = loadImage('data/03.svg');
  modules[4] = loadImage('data/04.svg');
  modules[5] = loadImage('data/05.svg');
  modules[6] = loadImage('data/06.svg');
  modules[7] = loadImage('data/07.svg');
  modules[8] = loadImage('data/08.svg');
  modules[9] = loadImage('data/09.svg');
  modules[10] = loadImage('data/10.svg');
  modules[11] = loadImage('data/11.svg');
  modules[12] = loadImage('data/12.svg');
  modules[13] = loadImage('data/13.svg');
  modules[14] = loadImage('data/14.svg');
  modules[15] = loadImage('data/15.svg');

}
draw tool. draws a specific module according to
its east, south, west and north neighbours.

MOUSE
drag left           : draw new module
drag right          : delete a module

KEYS
del, backspace      : clear screen
g                   : toggle show grid
d                   : toggle show module values
s                   : save png