Question regarding compiling Elixir pattern to MLIR (PDL dialect)

pattern-gen.ex

# I’m working on compiling Elixir pattern to MLIR (the PDL dialect),
# so that it could be used to manipulate another piece of IR.

# If we want to rewrite
res = TOSA.add(a, b) >>> Type.ranked_tensor([2, 3], Type.f32())
res1 = TOSA.add(res, b) >>> Type.ranked_tensor([2, 3], Type.f32())
# to
res1 = TOSA.sub(res, b) >>> Type.ranked_tensor([2, 3], Type.f32())

# We can declare a pattern like this
pattern replace_multi_add_op(
          %TOSA.Add{operands: [a, b], results: [res]},
          _t = %TOSA.Add{
            operands: [
              {:bound, res},
              {:bound, b}
              # the code is working now (including the codegen and IR manipulation), 
              # but I am wondering how to not use the :bound here? 
              # there should be a more elegant way to prevent generating IR for this unbound variable
              # I have tried to use ^res but it is not working
              # Also any advice on the general pattern expression (especially regarding nesting) is appreciated.
            ],
            results: [res1]
          }
        ) do
  %TOSA.Sub{operands: [a, b]}
end

diff in the broader context: https://github.com/beaver-project/beaver/compare/add-prototype-dsl?expand=1#diff-cbf34ba9f03a1facb5709bcc0887a1a32984b0ecb40c0215816095de6b6c6a9dR128-R131