+ If a module with the same NAME is already known, then it is returned
+ unchanged: the BODY is not evaluated.
+
+ The LOCATION may be any printable value other than `t' (though
+ `file-location' objects are most usual) indicating what provoked this
+ module definition: it gets reported to the user if an import cycle is
+ detected. This check is made only if a TRUENAME is supplied.
+