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Carries out a depth-first traversal of this syntax tree, yielding
every Sexp of type `target_type`. The traversal ignores any node
whose type is listed in the Array `ignoring`.
Takes a block as well.

target_type - the type to look for, e.g. :send, :block
ignoring - types to ignore, e.g. [:casgn, :class, :module]
blk - block to execute for every hit. Gets passed in the matching element itself.

  context.each_node(:send, [:mlhs]) do |call_node| .... end
  context.each_node(:lvar).any? { |it| it.var_name == 'something' }

Returns an array with all matching nodes.
TODO: without a block, this doesn't do what one might expect
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