I noticed that in many documents you encounter `$f: X \to Y$` instead of `$f \colon X \to Y$`. Another example are quantified expressions: `$\forall x: P (x)$` versus `$\forall x \colon P(x)$`. I find the `\colon` visually more pleasing, however I sometimes think I seem to be the only one, given the overwhelming amount of documents that seem to use `:`. Is there a rule of thumb to decide where you should use either `\colon` or `:`?