Added test function to compare syntax trees

tst/testinstall/syntaxtree.tst

@@ -10,6 +10,13 @@ gap> SyntaxTree(x -> x);
 <syntax tree>
 gap> SyntaxTree(\+);
 Error, SYNTAX_TREE: <func> must be a plain GAP function (not a function)
+gap> test_tree := function( f )
+>       local curr_tree, new_func, new_tree;
+>       curr_tree := SYNTAX_TREE( f );
+>       new_func := SYNTAX_TREE_CODE( curr_tree );
+>       new_tree := SYNTAX_TREE( new_func );
+>       return new_tree = curr_tree;
+> end;;
 
 # Just try compiling all functions we can find in the workspace
 # to see nothing crashes.
@@ -37,8 +44,7 @@ gap> testit := function(f)
 >   local tree;
 >   tree := SYNTAX_TREE(f);
 >   Display(tree);
->   # TODO: recode the tree, then decode again and compare
->   return true;
+>   return test_tree( f );
 > end;;
 
 # T_PROCCALL_0ARGS
@@ -2687,6 +2693,23 @@ rec(
   type := "T_FUNC_EXPR",
   variadic := false )
 true
+gap> testit( x -> [, [] ] );
+rec(
+  nams := [ "x" ],
+  narg := 1,
+  nloc := 0,
+  stats := rec(
+      statements := [ rec(
+              obj := rec(
+                  list := [ , rec(
+                          list := [  ],
+                          type := "T_LIST_EXPR" ) ],
+                  type := "T_LIST_EXPR" ),
+              type := "T_RETURN_OBJ" ) ],
+      type := "T_SEQ_STAT" ),
+  type := "T_FUNC_EXPR",
+  variadic := false )
+true
 
 # T_LIST_TILDE_EXPR
 gap> testit(x -> [~]);