Avoid or clarify possible pun on Base.parent

[goretkin]

AbstractTrees.jl/examples/binarytree_infer.jl

Line 36 in babf381

Base.parent(root::BinaryNode, node::BinaryNode) = isdefined(node, :parent) ? node.parent : nothing

The definition doesn't seem to be used, and I'm not sure what it illustrates. In any case, this seems to be a pun on Base.parent:

help?> parent
[...]
  parent(A)

  Return the underlying "parent array”. This parent array of objects of types SubArray, ReshapedArray or LinearAlgebra.Transpose is what was passed as an argument to view, reshape,
  transpose, etc. during object creation. If the input is not a wrapped object, return the input itself.

[jlumpe]

The parent API hasn't really been strictly defined yet, so I agree that Base.parent shouldn't be used for getting the parent of a node. I plan to submit a PR for that sometime in the near future in which it's a separate function.