Do not allow elements to immediately nest inside themselves.

Plain list is omitted because they can nest.

src/parser/greater_block.rs

@@ -13,6 +13,7 @@ use crate::parser::parser_with_context::parser_with_context;
 use crate::parser::util::blank_line;
 use crate::parser::util::exit_matcher_parser;
 use crate::parser::util::get_consumed;
+use crate::parser::util::immediate_in_section;
 use crate::parser::util::maybe_consume_trailing_whitespace_if_not_exiting;
 use crate::parser::util::start_of_line;
 use crate::parser::Element;
@@ -35,6 +36,12 @@ pub fn greater_block<'r, 's>(
     context: Context<'r, 's>,
     input: &'s str,
 ) -> Res<&'s str, GreaterBlock<'s>> {
+    // TODO: Do I need to differentiate between different greater block types.
+    if immediate_in_section(context, "greater block") {
+        return Err(nom::Err::Error(CustomError::MyError(MyError(
+            "Cannot nest objects of the same element",
+        ))));
+    }
     start_of_line(context, input)?;
     let (remaining, _leading_whitespace) = space0(input)?;
     // TODO: Not handling indentation before start of block
@@ -49,6 +56,7 @@ pub fn greater_block<'r, 's>(
     let (remaining, _nl) = line_ending(remaining)?;
     let parser_context = context
         .with_additional_node(ContextElement::ConsumeTrailingWhitespace(true))
+        .with_additional_node(ContextElement::Context("greater block"))
         .with_additional_node(ContextElement::GreaterBlock(name))
         .with_additional_node(ContextElement::ExitMatcherNode(ExitMatcherNode {
             class: ExitClass::Alpha,