`\phiAgain` in LaTeX output

[yegor256]

After #193 is implemented.

I'm doing this:

$ cat foo.phi
{⟦ m ↦ Q.k, k ↦ ⟦⟧ ⟧.k}
$ phino rewrite --normalize --sweet --log-level=DEBUG < foo.phi
[DEBUG]: Amount of rewriting cycles: 25
[DEBUG]: Reading from stdin
[DEBUG]: The --normalize option is provided, built-it normalization rules are used
[DEBUG]: Starting rewriting cycle 0 out of 25
[DEBUG]: DOT
{⟦ ρ ↦ ∅ ⟧(
  ρ ↦ ⟦
    m ↦ Φ.k,
    k() ↦ ⟦⟧
  ⟧
)}
[DEBUG]: Starting rewriting cycle 1 out of 25
[DEBUG]: COPY
{⟦
  ρ ↦ ⟦
    m ↦ Φ.k,
    k() ↦ ⟦⟧
  ⟧
⟧}
[DEBUG]: Starting rewriting cycle 2 out of 25
[DEBUG]: Rewriting is stopped since it does not affect program anymore
[DEBUG]: Printing rewritten 𝜑-program as phi
{⟦
  ρ ↦ ⟦
    m ↦ Φ.k,
    k() ↦ ⟦⟧
  ⟧
⟧}

Here, at the second step (COPY) we print the expression that contains something that has already been seen in the previous step (DOT):

⟦ m ↦ Φ.k, k() ↦ ⟦⟧ ⟧

If --output=latex (#193) is specified, we should use \phiAgain to highlight the places that have been seen earlier (with a unique marker of them):

{⟦ ρ ↦ ∅ ⟧(
  ρ ↦ \phiAgain{1}{⟦
    m ↦ Φ.k,
    k() ↦ ⟦⟧
  ⟧}
)}

{⟦
  ρ ↦ \phiAgain{1}{⟦
    m ↦ Φ.k,
    k() ↦ ⟦⟧
  ⟧}
⟧}