Syntax: {{tmpl|arg|arg|...}} to include. {{{n}}} for argument.

S  = S1 | {{{1}}}
S1 = S2 | {{{1}}} | {{{2}}}
S2 = {{ {{ {{{1}}} | {{{3}}} }} | {{ {{{2}}} | {{{3}}} }} }}
K  = I {{{1}}}
I  = {{{1}}}

Unfortunately MW recognizes the text in these bodies as literal, thus does not
evaluate them as template code. So -- what to do?