Arbitrary information, as defined in
Trf_TypeDefinition.clientData.
DESCRIPTION
The procedure takes the specified optionname (rejecting
illegal ones) and places the given optionvalue into the
container. All necessary conversions from a Tcl_Obj to the
required type should be done here. Return value is a standard
tcl error code. In case of failure and interp not NULL an
error message should be left in the result area of the
specified interpreter. This procedure makes sense for tcl
version 8 and above only.