TSTART = "{{"
TEND = "}}"
VSTART = "{{{"
VEND = "}}}"
DSTART = "{'"
DEND = "'}"
PIPE = "|"
PIPES = "||"
OUTERTEXT = anything, except for TSTART, or DSTART
INNERITEXT = anything, except for TSTART, DSTART, VSTART, PIPE(s), TEND
INNERDTEXT = anything, except for TSTART, DSTART, VSTART, PIPE(s), DEND
BODYTEXT = anything, except TSTART, DSTART, VSTART, DEND
VNAME = anything, except for PIPE(s), VEND

<program> ::= (OUTERTEXT | <invoke> | <define>)*
<invoke> ::= TSTART <itext> <targs> TEND
<targs> ::= (PIPE <itext>?)*
<itext> ::= (INNERITEXT | <tvar> | <invoke> | <define> )*
<tvar> ::= VSTART VNAME (PIPE <itext>)? VEND

<define> ::= DSTART <dtextn> <dparams> PIPES <dtextb> DEND
<dtextn> ::= (INNERDTEXT | <invoke> | <define> | <tvar>)*
<dparams> ::= (PIPE <dtextp>)*
<dtextp> ::= (INNERDTEXT | <invoke> | <define> | <tvar>)+
<dtextb> ::= (BODYTEXT | <invoke> | <define> | <tvar>)*