Issue warning for empty units during semantic analysis about nmodl HOT 4 OPEN

Can you give an example here of such files

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On top of that it would be useful to have a more detailed description about what empty units are what they are used for

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If by empty, you mean no units specified, then that is equivalent to dimensionless, i.e. units syntax of (1).

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About empty units in nrn.
The truely empty units () is forbidden because:

Redefinition of units () to:	0.001 m2-kg/sec2-coul
 is ignored.
They remain:	1
Error: Units redefinition at line 3 in file a.mod
    () = (millivolt)
                   ^

The onespace unit ( ) is accepted once.

Inside function arguments both are accepted.

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