foo.sed

:on_stream
    /^0	\(.*\)$/  { b on_document }
    n
    b on_stream

:on_document
    s/^.*$/DOC1 &/;p
    n
:on_document_back
    /^[0-9][0-9]*	\(.*\)$/  { b on_body }
    b on_stream

:on_body
    s/^.*$/TXT1 &/;p
    n
:on_body_back
    /^0	\(.*\)$/  { b on_document }
    /^[0-9][0-9]*		\(.*\)$/ { b on_code_indented }
    /^[0-9][0-9]*	\(~~~\|```\)$/     { h ; b on_code_fenced }
    /^[0-9][0-9]*	\(\[\]\:\)\([a-zA-Z0-9]*\)\s*(*\([^)]*\)\s*)*\s*$/   { b on_reference }
    /^[0-9][0-9]*	\(.*\)$/  { b on_body }
    b on_stream

:on_reference
s/^.*$/REF1 &/;p
n
    b on_body_back

:on_code_indented
    s/^.*$/PRE1 &/;p
    n
:on_code_indented_back
    /^[0-9][0-9]*		\(.*\)$/ { b on_code_indented }
    /^[0-9][0-9]*	$/       { b on_code_indented }
    /^[0-9][0-9]*	\(.*\)$/  { b on_body }
    b on_stream

:on_code_fenced
    s/^.*$/PRE2 &/;p
    n
:on_code_fenced_back
    /^[0-9][0-9]*	\(~~~\|```\)$/ {
        G

        /^[0-9][0-9]*	~~~\
[0-9][0-9]*	~~~$/      { b on_code_fenced_close }

        /^[0-9][0-9]*	```\
[0-9][0-9]*	```$/   { b on_code_fenced_close }

        s/^\([0-9][0-9]*	\)\(~~~\|```\).*$/\1\2/

        b on_code_fenced
    }
    b on_code_fenced

:on_code_fenced_close
        s/^\([0-9][0-9]*	\)\(~~~\|```\).*$/\1\2/
s/^.*$/PRE3 &/;p
n
b on_body_back

foo.src

			$(fnc stream)
			    ${find_document}  { $(jmp document) }
			    n
			    $(jmp stream)

			$(def document DOC1)
			    ${find_anything}  { $(jmp body) }
			    $(jmp stream)

			$(def body TXT1)
			    ${find_document}  { $(jmp document) }
			    ${find_codeblock} { $(jmp code_indented) }
			    ${find_fence}     { h ; $(jmp code_fenced) }
			    ${find_context}   { $(jmp reference) }
			    ${find_anything}  { $(jmp body) }
			    $(jmp stream)

			$(fnc reference)
				$(dmp REF1)
				n
			    $(jmp body back)

			$(def code_indented PRE1)
			    ${find_codeblock} { $(jmp code_indented) }
			    /^${line}$/       { $(jmp code_indented) }
			    ${find_anything}  { $(jmp body) }
			    $(jmp stream)

			$(def code_fenced PRE2)
			    ${find_fence} {
			        G

			        /^${line}~~~\\
					${line}~~~$/      { $(jmp code_fenced_close) }

			        /^${line}\`\`\`\\
					${line}\`\`\`$/   { $(jmp code_fenced_close) }

			        ${coalesce_fence}

			        $(jmp code_fenced)
			    }
			    $(jmp code_fenced)

			$(fnc code_fenced_close)
		        ${coalesce_fence}
				$(dmp PRE3)
				n
				$(jmp body back)