PathSpec

PathSpec : Join | Simple .
Join = "(join" <source> PathSpec <target> PathSpec ")".
Simple = "->" <source> Node <target> Node.

satisfies(p,ps) if p is an expansion of ps.

p satisfies R(p,ps)  
ps = Simple(n1,n2):
source(p) = source(ps)=n1
target(p) = target(ps)=n2

ps = Join(ps1,ps2):
well-formed: target(ps1) = source(ps2)
source(p) = source(ps1)
target(p) = target(ps2)
p contains target(ps1)

ps =