Hard names aside, might it be more logical to write it like this

   diffuse {<amount> method <bla> roughness <x> ...}

similar to e.g. how reflection or media parameters are grouped.
Supported parameters depend on the method, and if it makes sense
to have e.g. a roughness parameter for multiple methods it can
have different range / interpretation depending on method).

You can also make method name lambert explicit this way except
method lambert will be default anyway.

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