FEAT: allow use of series index in parse's remove command

resolves: Oldes/Rebol-issues#2541

Author: Oldes

src/core/u-parse.c

@@ -966,6 +966,23 @@ void Print_Parse_Index(REBCNT type, REBVAL *rules, REBSER *series, REBCNT index)
 
 					case SYM_REMOVE:
 						SET_FLAG(flags, PF_REMOVE);
+						if (IS_WORD(rules)) {
+							// optionally using stored index as a counter
+							// https://github.com/Oldes/Rebol-issues/issues/2541
+							item = Get_Var(rules);
+							if (VAL_SERIES(item) == parse->series) {
+								rules++;
+								if (VAL_INDEX(item) < index) {
+									begin = VAL_INDEX(item);
+									count = index - VAL_INDEX(item);
+								}
+								else {
+									begin = index;
+									count = VAL_INDEX(item) - index;
+								}
+								goto do_remove;
+							}
+						}
 						continue;
 
 					case SYM_INSERT:
@@ -1407,6 +1424,7 @@ void Print_Parse_Index(REBCNT type, REBVAL *rules, REBSER *series, REBCNT index)
 					Throw_Return_Series(parse->type, ser);
 				}
 				if (GET_FLAG(flags, PF_REMOVE)) {
+do_remove:
 					if (count) {
 						if (IS_PROTECT_SERIES(series)) Trap0(RE_PROTECTED);
 						Remove_Series(series, begin, count);

src/tests/units/parse-test.r3

@@ -551,6 +551,32 @@ Rebol [
 	--assert parse v: [#L 1 "a" #L 2 "b"][some [string! | lr]]
 	--assert v = ["a" "b"]
 
+--test-- "remove using series' index"
+;@@ https://github.com/Oldes/Rebol-issues/issues/2541
+	--assert all [
+		parse s: "abcd" [skip p: 2 skip remove p skip]
+		s == "ad"
+	]
+	--assert all  [
+		parse s: "abcd" [skip (p: tail s) remove p]
+		s == "a"
+	]
+	--assert all [
+		parse s: [a b c d] [skip p: 2 skip remove p skip]
+		s == [a d]
+	]
+	--assert all  [
+		parse s: [a b c d] [skip (p: tail s) remove p]
+		s == [a]
+	]
+	;; here the word `p` is used, but not as an index, but as a value
+	--assert all  [
+		p: "bc" parse s: "abcd" [skip remove p skip]
+		s == "ad"
+	]
+	p: "xx"
+	--assert not parse s: "abcd" [skip remove p skip]
+
 --test-- "while .. remove"
 	remove-any-y: [while [remove #"y" | #"x"]] 
 	--assert parse v: "" remove-any-y