rule := target arrow source [options]

target ::= (quoted_string | #id | rect | point)
arrow  ::= ("->"|"=>"|"=")
source ::= (quoted_string | "this")

rect   ::= "rect" "(" number "," number "," number "," number ")"
point  ::= "point" "(" number "," number ")"

options ::= [position] [margin] [scale]

position ::= "position" ":" px ["," py]
scale    ::= "scale"    ":" ("fit" | scaledim ["," scaledim])
margin   ::= "margin"   ":" number ["," number ["," number "," number]]

px ::= number | perc | "center" | "c" | "left" | "l" | "right" | "r" | "inherit" | "i"
py ::= number | perc | "center" | "c" | "top" | "t" | "bottom" | "b"
scaledim ::= number | perc | width | height | "uniform" | "length" number

perc   ::= number "%"
width  ::= "width"  "(" (#id | "this") ")"
height ::= "height" "(" (#id | "this") ")"

comment ::= "%" any number of chars NEWLINE