code: public void join(String aSep) {
          List<Object> newChildren;
          int i;
          int len = this.children.size();
          if (len > 0) {
              newChildren = new ArrayList<>();
              for (i = 0; i < len - 1; i++) {
                  newChildren.add(this.children.get(i));
                  newChildren.add(aSep);
              }
              newChildren.add(this.children.get(i));
              this.children = newChildren;
          }
      }
docs: Like `String.prototype.join` except for SourceNodes. Inserts `aStr` between each of `this.children`.
  
  @param aSep
  The separator.