code: function(sbmA, sbmB) {
      // check if size compatible
      if (sbmA.y !== sbmB.x) {
        return
      }
      // create a new matrix
      return this.make((function (i, j) {
        return _.reduce(_.map(_.range(sbmA.y), (function(id) {
          return this.check(sbmA, i, id) && this.check(sbmB, id, j)
        }).bind(this)), function (sum, a) {
          return sum ^ a
        }) == 1
      }).bind(this), { x: sbmA.x, y: sbmB.y })
    }
docs: multiply two matrices