(7) Document the @placeholders@ available to the various C code fragments.

(8) Note that we can use #define's to make the class and instance
    types available as fixed named without resorting to placeholders,
    and still keeping it in a local scope.

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idea: critcl::cproc -- api to register return types, argument types

      argument types: conversion from Tcl_Obj*, type checking (values)
      return types:   conversion to Tcl_Obj*, (type/value checking ?)