Inline app3/_app3/nodes code.

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With the current implementation we have assert(3 <= list.length && list.length <= 12) in src/0-core/concatenate/nodes.js.

Plan:

• Write tests that cover all cases in nodes (from list.length = 3 to list.length = 12) 38b9e05
• Write a benchmark that stresses the concat method
• Profile current implementation
• If any of nodes, _app3, app3 shows up as a bottleneck, profile these implementations:
  1. unroll loop iterations in nodes (max iterations is fixed)
  2. switch on list.length in nodes 7362602
  3. handle all cases separately as Digit methods and remove nodes: First implicit branch on the protototype of B, then implicit branch on the prototype of A (e.g. A._nodes_from_list_and_Two(list, B) where B instanceof Two), and finally explicit branch on list.length using a switch

Also the current base cases in app3 use the generic _append and _prepend where list has a size between 1 and 4.

• Explore unrolling these generic loops in those cases.

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For ii: If implementing the default branch generically we could use nodes to create any Tree from scratch (see #108).

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iii. is actually Exercise 3 (page 9) in Hinze and Paterson (see README for link).