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I think this would be useful for selecting predefined values.
checkbox and drop-down menu is already there:
# A checkbox is a boolean value:
param(:ab, TypeBoolean, "A bool")
# Every parameter can be given a "choice" (a list of description/value pairs).
# This will make it become displayed as drop-down menu.
param(:ac, TypeInt, "A choice", :choices => [ [ "Choice A", 1 ], [ "Choice B", 2 ] ])