Ftyjtk

I have a relation symbol that I want to define through an equality. The problem is that when I do this, LaTeX puts too little space between the relation symbol and the equality symbol. My guess is that this is because both are relation symbols. Here is a MWE:

documentclass{article}

begin{document}

  [cong_{mathcal{F}} = {(x,y) in X^2 mid forall F in mathcal{F}. x in F iff y in F}.]

end{document}

If I specify that = should be a binary operator, then the spacing between those two symbols is correct, but now the spacing between equality and the following set is too small:

documentclass{article}

begin{document}

  [cong_{mathcal{F}} mathbin{=} {(x,y) in X^2 mid forall F in mathcal{F}. x in F iff y in F}.]

end{document}

So my question is: How do I ensure that the spacing around = is correct on both sides? I could try hacking it by inserting spacing manually until I'm happy, but I am hoping for a more standardised answer that will also work in other situations.

Just brace the symbol:

documentclass{article}

begin{document}



[

{cong_{mathcal{F}}} = {(x,y) in X^2 mid forall F in mathcal{F}. x in F iff y in F}.  

]



end{document}

What happens is that TeX adds no space between consecutive relation atoms (cong_{mathcal{F}} and =), but when a symbol is braced it becomes an “ordinary atom”, just like a letter.