Do not allow elements to immediately nest inside themselves.

Plain list is omitted because they can nest.

src/parser/footnote_definition.rs

@@ -2,6 +2,8 @@ use super::error::Res;
 use super::util::WORD_CONSTITUENT_CHARACTERS;
 use super::Context;
 use crate::parser::element::element;
+use crate::parser::error::CustomError;
+use crate::parser::error::MyError;
 use crate::parser::exiting::ExitClass;
 use crate::parser::greater_element::FootnoteDefinition;
 use crate::parser::parser_context::ContextElement;
@@ -10,6 +12,7 @@ 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;
 use crate::parser::util::maybe_consume_trailing_whitespace_if_not_exiting;
 use crate::parser::util::start_of_line;
@@ -30,12 +33,18 @@ pub fn footnote_definition<'r, 's>(
     context: Context<'r, 's>,
     input: &'s str,
 ) -> Res<&'s str, FootnoteDefinition<'s>> {
+    if immediate_in_section(context, "footnote definition") {
+        return Err(nom::Err::Error(CustomError::MyError(MyError(
+            "Cannot nest objects of the same element",
+        ))));
+    }
     start_of_line(context, input)?;
     // Cannot be indented.
     let (remaining, (_lead_in, lbl, _lead_out, _ws)) =
         tuple((tag_no_case("[fn:"), label, tag("]"), space0))(input)?;
     let parser_context = context
         .with_additional_node(ContextElement::ConsumeTrailingWhitespace(true))
+        .with_additional_node(ContextElement::Context("footnote definition"))
         .with_additional_node(ContextElement::ExitMatcherNode(ExitMatcherNode {
             class: ExitClass::Alpha,
             exit_matcher: &footnote_definition_end,