code: function eltCompare(elt1, elt2) {
    return (elt1.c == elt2.c && elt1.r == elt2.r);
  }
docs: Compare two objects defining row/col locations in an array
  and return true if they represent the same row/col location.
  @private
  @param {ArrayLoc} elt1
  @param {ArrayLoc} elt2
  @return {boolean} - Whether or not these two array locations
  represent the same row/column.