code: public static VariableNumMap unionAll(Collection<VariableNumMap> varNumMaps) {
      VariableNumMap curMap = EMPTY;
      for (VariableNumMap varNumMap : varNumMaps) {
        curMap = curMap.union(varNumMap);
      }
      return curMap;
    }
docs: Returns the union of all of the passed-in maps, which may not
  contain conflicting mappings for any variable number.
  
  @param varNumMaps
  @return