The list operator '<<' (append) and '>>' (prepend) have been replaced

with the new operators '<+' and '+>' respectively.
'<<' and '>>' are now used only for left and right binary shit
respectively. Further, their priority has been increased moving them to
a higher priority than that of the assignment operator.

symbol.go

@@ -60,8 +60,8 @@ const (
 	SymQuestionExclam                    //  49: '?!'
 	SymDoubleAt                          //  50: '@@'
 	SymDoubleColon                       //  51: '::'
-	SymInsert                            //  52: '>>'
-	SymAppend                            //  53: '<<'
+	SymDoubleGreater                     //  52: '>>'
+	SymDoubleLess                        //  53: '<<'
 	SymCaret                             //  54: '^'
 	SymDollarRound                       //  55: '$('
 	SymOpenClosedRound                   //  56: '()'
@@ -71,6 +71,8 @@ const (
 	SymStarEqual                         //  60: '*='
 	SymSlashEqual                        //  61: '/='
 	SymPercEqual                         //  62: '%='
+	SymPlusGreater                       //  63: '+>'
+	SymLessPlus                          //  64: '<+'
 	SymChangeSign
 	SymUnchangeSign
 	SymIdentifier