When assigning instance attributes in Maya, they can be named like Primvars, following the form: rmanTvarname, where T represets the type, one of 'F', 'C', 'V', 'N', 'P', 'H', 'M', 'S', 'I', T can alternatively be an underscore, in which case the type is inferred.

Or they can be named like optional riattrs: rman__riattr__namespace_varname (eg. rman__riattr__user_foo)
The latter format infers the type based on the attribute type in Maya, and it also can be used to add arbitrary optional riattrs, eg. rman__riattr__trace_maxspeculardepth

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