tl: extend F[n:m] and G[n:m] to the case of m=$

Suggested by Victor Khomenko.

* spot/tl/formula.cc, spot/tl/formula.hh, spot/parsetl/parsetl.yy:
Implement this.
* NEWS, doc/tl/tl.tex: Document it.
* tests/core/sugar.test, tests/python/ltlparse.py: Add some tests.

doc/tl/tl.tex

@@ -523,7 +523,7 @@ $\M$, and $\W$ if you are only familiar with $\X$ and $\U$.
 \subsection{Syntactic Sugar}
 
 The syntax on the left is equivalent to the syntax on the right.
-These rewritings taken from the syntax of TSLF~\citep{jacobs.16.synt}
+Some of rewritings taken from the syntax of TSLF~\citep{jacobs.16.synt}
 are performed from left to right when parsing a formula.  They express
 the fact that some formula $f$ has to be true in $n$ steps, or at some
 or all times between $n$ and $m$ steps.
@@ -532,9 +532,11 @@ or all times between $n$ and $m$ steps.
   \XREP{\mvar{n}} f
   & \equiv \underbrace{\X\X\ldots\X}_{\mathclap{\text{\mvar{n} occurrences of~}\X}} f \\
   \FREP{\mvar{n}:\mvar{m}}f
-  & \equiv \underbrace{\vphantom{(}\X\X\ldots\X}_{\mathclap{\text{\mvar{n} occ. of~}\X}} (f \OR \underbrace{\X(f \OR \X(\ldots \OR \X f))}_{\mathclap{\mvar{m}-\mvar{n}\text{~occ. of~}\X}}) \\
+  & \equiv \underbrace{\vphantom{(}\X\X\ldots\X}_{\mathclap{\text{\mvar{n} occ. of~}\X}} (f \OR \underbrace{\X(f \OR \X(\ldots \OR \X f))}_{\mathclap{\mvar{m}-\mvar{n}\text{~occ. of~}\X}})
+  & \FREP{\mvar{n}:}f &\equiv \XREP{\mvar{n}}\F{}f\\
   \GREP{\mvar{n}:\mvar{m}}f
-  & \equiv \underbrace{\vphantom{(}\X\X\ldots\X}_{\mathclap{\text{\mvar{n} occ. of~}\X}} (f \AND \underbrace{\X(f \AND \X(\ldots \AND \X f))}_{\mathclap{\mvar{m}-\mvar{n}\text{~occ. of~}\X}}) \\
+  & \equiv \underbrace{\vphantom{(}\X\X\ldots\X}_{\mathclap{\text{\mvar{n} occ. of~}\X}} (f \AND \underbrace{\X(f \AND \X(\ldots \AND \X f))}_{\mathclap{\mvar{m}-\mvar{n}\text{~occ. of~}\X}})
+  & \GREP{\mvar{n}:}f &\equiv \XREP{\mvar{n}}\G{}f\\
 \end{align*}
 
 \subsection{Trivial Identities (Occur Automatically)}