private static void copyLine(DiffLines from, DiffLines to, int index) {
		ValidatableLine line = from.get(index);
		if (line instanceof PlaceHolderLine) {
			throw new AssertException(
				"copyLine() is meant to copy only real lines, not placeholders");
		}
		to.add(line);
	}
Copies from the source to the destination all lines that are not placeholders.  If the 
two input lists are not the same after the copy, then the smaller list is padded with
placeholder lines.
Ghidra/Features/Base/src/main/java/ghidra/app/util/html/diff/DataTypeDiffBuilder.java