Formal notation for a required statement Is there a formal notation to distinguish an equality that is a true statement, e.g.,

[...] and hence,
  $$
    x = x^2 - 1
  $$
  [...]

from a demand, e.g.,

[...] so we require
  $$
  x \stackrel{!}{=} x^2 - 1
$$
  [...]

?
The same thing could apply to membership to sets $x\stackrel{!}{\in}\mathbb{R}$ and more.
I've seen the exclamation mark syntax once, and I faintly remember having seen some other notation, but I'm not sure if any of this is commonly used.
 A: Basically  if I were you I would write the exclamation mark. If you fear that it is not understandable then remark in your text that this should specify that it is a demand and not a statement.
I think a lot of mathematicians use the exclamation mark in the sense you think of it. However, I don't think that it is "official" notation (like $e$ for the Euler number or so...).

(Although it doesn't exactly go for the question itself as the following deals with statements rather than demands I want to shortly mention here an)
Interesting note: Frege (the founder of modern logic)  introduced a special sign to indicate that what follows it is an assertion rather than a truth. This sign is "$\vdash$".
So he would write at the beginning of a proof: 
$\vdash 1+1=2$ in $\mathbb{R}$.
To say that he states that $1+1=2$ in $\mathbb{R}$ is true.
Contrariwise 
$1+1=2$ in $\mathbb{R}$,
is for Frege a truth value (or to be more accuarte: the Truth itself.)
Actually people in mathematical logic use this very sign to indicate tautologies. However, I don't really know if there is any connection between this use and Frege...
