val genr_function :
Otype.constructor_definition -> Genr_base.generated_functions
genr_function ggenerates a unary function that normalizes terms that are built with
gas their head constructor.
We perform here the generation of code for the construction function only in the case of a ``true'' unary generator.
A true unary generator has one argument and this argument is
not of type
list. Hence, this argument should not be treated as the
variable length list of arguments of an associative binary operation.
The case of an associative binary operator is handled in the module
Genr_listary; we branch to this case when encountering an associative
operator, since a variable length list of argument cannot exist for a
true unary operator.