public static Subst Lookup(SpeciesFlow var, Lst <Subst> substs) // returns null for not found
{
if (substs is Cons <Subst> cons)
{
if (var.SameSpecies(cons.head.var))
{
return(cons.head);
}
else
{
return(Lookup(var, cons.tail));
}
}
else
{
return(null);
}
}
