Merge branch 'develop' into #268/c++20

src/program.h

@@ -223,6 +223,7 @@ class Program : public deque<Object*>, public Lexer, public Object {
     void RpnStrsize();
     void RpnStrpos();
     void RpnStrsub();
+    void RpnEndl();
 
     // test-core
     void RpnTest();

src/rpn-string.cc

@@ -94,3 +94,9 @@ void Program::RpnStrsub() {
     stack_.push(new String(stack_.value<String>(2).substr(first, len)));
     stack_.erase(1, 3);
 }
+
+/// @brief endline character
+///
+void Program::RpnEndl() {
+    stack_.push_front(new String("\n"));
+}

test/060-string.md

@@ -305,3 +305,15 @@
 -> stack should be "sub"
 
 `del`
+
+## endl 1
+`endl type`
+
+-> stack should be "string"
+
+`del`
+
+## endl 2
+`endl num`
+
+-> stack should be 10