grrr ::= (nil) | grrr block
block ::= rule-block | data-block | decl-block
rule-block ::= "rule section:" rules
data-block ::= "data section:" data
decl-block ::= "declarations:" decls

rules ::= (nil) | rules rule
rule ::= pattern "=>" pattern ";"
pattern ::= node-chain | pattern "," node-chain
node-chain ::= precond nodes postcond
nodes ::= node | nodes "-" node
precond ::= (nil) | node "!"
postcond ::= (nil) | "!" node
node ::= color | ":" name | color ":" name
color ::= cdata | color "*" cdata
name ::= cdata

data ::= (nil) | data datum
datum ::= pattern ";"