Implemented the sep helper.

js/test_cases/helpers_sep/main.dust

@@ -1,13 +1,29 @@
+Tags inside a sep{~n}
+================={~n}
 {#people}
     {name}{@sep petname="fluffy"},{pet}{petname}{/sep}
 {/people}{~n}
 
+sep inside a scalar{~n}
+==================={~n}
 {#scalar}
     {name}{@sep petname="fluffy"},{pet}{petname}{/sep}
 {/scalar}{~n}
 
+Nested sep inside another non-array section{~n}
+==========================================={~n}
 {#people}
     {#toys}
         {name}'s pet {pet} plays with a {.}{@sep}, {/sep}
     {/toys}
 {/people}{~n}
+
+Else block inside a sep{~n}
+======================={~n}
+{#people}
+    {name}{@sep},{pet}{petname}{:else}elseblock{/sep}
+{/people}{~n}
+
+Sep outside any section{~n}
+======================={~n}
+{@sep}sep is printed outside any section{/sep}

src/renderer/renderer.rs

@@ -2,6 +2,7 @@ use crate::parser::template;
 use crate::parser::Body;
 use crate::parser::DustTag;
 use crate::parser::Filter;
+use crate::parser::OwnedLiteral;
 use crate::parser::PartialNameElement;
 use crate::parser::Path;
 use crate::parser::Special;
@@ -645,7 +646,54 @@ impl<'a> DustRenderer<'a> {
                     }
                 }
             }
-            DustTag::DTHelperSep(parameterized_block) => {}
+            DustTag::DTHelperSep(parameterized_block) => {
+                let new_breadcrumbs = self.new_breadcrumbs_partial(
+                    breadcrumbs,
+                    breadcrumbs,
+                    None,
+                    &parameterized_block.explicit_context,
+                );
+
+                let index = self.get(breadcrumbs, &vec!["$idx"]);
+                let len = self.get(breadcrumbs, &vec!["$len"]);
+                match (index, len) {
+                    (Err(_), _) | (_, Err(_)) => {
+                        return self.render_maybe_body(
+                            &parameterized_block.contents,
+                            new_breadcrumbs.as_ref().unwrap_or(breadcrumbs),
+                            blocks,
+                        )
+                    }
+                    (Ok(index_resolved), Ok(len_resolved)) => {
+                        // Iteration contexts use OwnedLiteral::LPositiveinteger
+                        let index_cast = index_resolved
+                            .get_context_element_reference()
+                            .to_any()
+                            .downcast_ref::<OwnedLiteral>();
+                        let len_cast = len_resolved
+                            .get_context_element_reference()
+                            .to_any()
+                            .downcast_ref::<OwnedLiteral>();
+                        match (index_cast, len_cast) {
+                            (
+                                Some(OwnedLiteral::LPositiveInteger(index_number)),
+                                Some(OwnedLiteral::LPositiveInteger(len_number)),
+                            ) => {
+                                if *index_number == len_number - 1 {
+                                    return Ok("".to_owned());
+                                } else {
+                                    return self.render_maybe_body(
+                                        &parameterized_block.contents,
+                                        new_breadcrumbs.as_ref().unwrap_or(breadcrumbs),
+                                        blocks,
+                                    );
+                                }
+                            }
+                            _ => return Ok("".to_owned()),
+                        }
+                    }
+                }
+            }
             DustTag::DTHelperFirst(parameterized_block) => {}
             DustTag::DTHelperLast(parameterized_block) => {}
         }