code: def _beforeBuildPaintColrLayers(self, dest, source):
      # Sketchy gymnastics: a sequence input will have dropped it's layers
      # into NumLayers; get it back
      if isinstance(
          source.get("NumLayers", None), collections.abc.Sequence
      ):
          layers = source["NumLayers"]
      else:
          layers = source["Layers"]
  
      # Convert maps seqs or whatever into typed objects
      layers = [self.buildPaint(l) for l in layers]
  
      # No reason to have a colr layers with just one entry
      if len(layers) == 1:
          return layers[0], {}
  
      if self.cache is not None:
          # Look for reuse, with preference to longer sequences
          # This may make the layer list smaller
          layers = self.cache.try_reuse(layers)
  
      # The layer list is now final; if it's too big we need to tree it
      is_tree = len(layers) > MAX_PAINT_COLR_LAYER_COUNT
      layers = build_n_ary_tree(layers, n=MAX_PAINT_COLR_LAYER_COUNT)
  
      # We now have a tree of sequences with Paint leaves.
      # Convert the sequences into PaintColrLayers.
      def listToColrLayers(layer):
          if isinstance(layer, collections.abc.Sequence):
              return self.buildPaint(
                  {
                      "Format": ot.PaintFormat.PaintColrLayers,
                      "Layers": [listToColrLayers(l) for l in layer],
                  }
              )
          return layer
  
      layers = [listToColrLayers(l) for l in layers]
  
      # No reason to have a colr layers with just one entry
      if len(layers) == 1:
          return layers[0], {}
  
      paint = ot.Paint()
      paint.Format = int(ot.PaintFormat.PaintColrLayers)
      paint.NumLayers = len(layers)
      paint.FirstLayerIndex = len(self.layers)
      self.layers.extend(layers)
  
      # Register our parts for reuse provided we aren't a tree
      # If we are a tree the leaves registered for reuse and that will suffice
      if self.cache is not None and not is_tree:
          self.cache.add(layers, paint.FirstLayerIndex)
  
      # we've fully built dest; empty source prevents generalized build from kicking in
      return paint, {}
  
imports: import collections
  import copy
  import enum
  from functools import partial
  from math import ceil, log
  from typing import Any, Dict, Generator, Iterable, List, Mapping, Optional, Sequence, Tuple, Type, TypeVar, Union
  from fontTools.misc.arrayTools import intRect
  from fontTools.misc.fixedTools import fixedToFloat
  from fontTools.misc.treeTools import build_n_ary_tree
  from fontTools.ttLib.tables import C_O_L_R_
  from fontTools.ttLib.tables import C_P_A_L_
  from fontTools.ttLib.tables import _n_a_m_e
  from fontTools.ttLib.tables import otTables
  from fontTools.ttLib.tables.otTables import ExtendMode, CompositeMode
  from .errors import ColorLibError
  from .geometry import round_start_circle_stable_containment
  from .table_builder import BuildCallback, TableBuilder
label_desc: Use image node_id 2 for calling the LayerListBuilder obj's underlying member method code with example usage: obj._beforeBuildPaintColrLayers(dest, source) and returns: paint,  dict, layers,  dict, layer, layers,  dict, self