While working on Github issue #1457, 
<https://github.com/metamath/set.mm/issues/1457>I encountered a situation 
in set.mm which illustrates this topic. It is the definition of `gsum` 
(~df-gsum), which must be placed far before the related theorems because 
the symbol `gsum` is used in definition ~df-prds already. 
And it is not sufficient to have the constant definitions "$c gsum $." and 
"cgsu $a class gsum $." earlier (for metamath.exe it is, for mmj2 it isn't: 
mmj2 would throw an error message like "Axiom df-gsum has failed the 
definitional soundness check. The previous axiom df-prds uses the symbol 
being defined in this axiom.").

And since ~df-gsum uses `0g`, its definitions (including ~df-0g) must occur 
even before ~df-gsum! 

I think this is really awkward. There are two possible solutions:

   - to allow a definition (~df-...) to be placed after the usage of the 
   corresponding symbol (and before it is really used, of course). But I think 
   this is exactly the opposite to what Mario proposes.
   - to revise the definitions of ~df-prds, ~df-imas, etc. (but I do not 
   know if this would be possible at all...)

Alexander

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