twoIota		::= Decl {";" Decl} ".".
Decl		::= Pragma | Alphabet | Event.

Pragma		::= "pragma" <<<implementation-specific>>>.

Alphabet	::= "alphabet" AlphabetName {"," SymbolName}.

Event		::= "event" EventDeclName {"," Property}.
Property	::= "causes" EventApplName ["after" TimeSpec] {WhenTerm}.

TimeSpec	::= RationalNumber TimeUnit.
TimeUnit	::= "ms" | "s" | "m" | "h" | "d".

WhenTerm	::= "when" EventName ">" EventName.

EventDeclName	::= EventDeclComp {EventDeclComp}.
EventDeclComp	::= SymbolName | "(" ParamName "=" MatchExpr ")".

MatchExpr	::= AlphabetName ["+"].

EventApplName	::= EventApplComp {EventApplComp}.
EventApplComp	::= SymbolName | "(" AlphabetExpr ")".

AlphabetExpr	::= AlphabetTerm {"|" AlphabetTerm}.
AlphabetTerm	::= "succ" ParamName
		  | "pred" ParamName
		  | "next" ParamName
		  | "prev" ParamName
		  | "first" AlphabetName
		  | "last" AlphabetName
		  | SymbolName
		  .