code: void copyFrom(CollationDataBuilder src, CEModifier modifier) {
          if(!isMutable()) {
              throw new IllegalStateException("attempt to copyFrom() after build()");
          }
          CopyHelper helper = new CopyHelper(src, this, modifier);
          Iterator<Trie2.Range> trieIterator = src.trie.iterator();
          Trie2.Range range;
          while(trieIterator.hasNext() && !(range = trieIterator.next()).leadSurrogate) {
              enumRangeForCopy(range.startCodePoint, range.endCodePoint, range.value, helper);
          }
          // Update the contextChars and the unsafeBackwardSet while copying,
          // in case a character had conditional mappings in the source builder
          // and they were removed later.
          modified |= src.modified;
      }
docs: Copies all mappings from the src builder, with modifications.
  This builder here must not be built yet, and should be empty.