Several functions use objects of type instantiation, consisting of type and
term instantiations and higher-order matching information. This instantiation
null_inst is the trivial instantiation that does nothing.
FAILURE CONDITIONS
Not applicable.
EXAMPLE
Instantiating a term with it has no effect:
# instantiate null_inst `x + 1 = 2`;;
val it : term = `x + 1 = 2`