custom equation numbering

[Ivan Werning]

How do I change an equation number that was chosen automatically from  
forcing a particular number or symbol?

That is, I want the analog of \tag in Latex
For example, I have an equation that is numbered "(5)" which is chosen  
by default because it comes after "(4)". That's the correct normal  
behavior of course. However, I want to break briefly out of the  
default and number this one equation "(3')" because it is a variant of  
a previous equation numbered "(3)".

Thanks!

-Ivan