Trivially reduce 'a[*1]' to 'a'.

* src/ltlast/bunop.cc (bunop::instance): Here.
* src/ltlast/bunop.hh, doc/tl/tl.tex: Document it.
* src/ltltest/equals.test: Test it.

src/ltlast/bunop.cc

@@ -211,6 +211,10 @@ namespace spot
 		return constant::empty_word_instance();
 	      }
 
+	    // - Exp[*1] = Exp
+	    if (min == 1 && max == 1)
+	      return child;
+
 	    // - Exp[*i..j][*min..max] = Exp[*i(min)..j(max)]
 	    //                                       if i*(min+1)<=j(min)+1.
 	    if (child->kind() == BUnOp)

src/ltlast/bunop.hh

@@ -51,6 +51,7 @@ namespace spot
       ///   - [*0][*min..max] = [*0]
       ///   - Exp[*0] = [*0]
       ///   - Exp[*i..j][*k..l] = Exp[*ik..jl] if i*(k+1)<=jk+1.
+      ///   - Exp[*1] = Exp
       ///
       /// These rewriting rules imply that it is not possible to build
       /// an LTL formula object that is SYNTACTICALLY equal to one of