message: List LayoutFunction segments invariants
diff: @@ -280,6 +280,8 @@ class LayoutFunction {
  return a.column < b.column;
  });
  }
  + // Elements in 'segments_' must have unique columns and be sorted by column.
  + // The first segment must start at column 0.
  // std::set would be more appropriate generally, but due to really
  // small amount of elements the container has to hold and ordered inserts, it
  // probably wouldn't help in anything.
  