# ==============
# Family cousins
# ==============

@prefix : <http://example.org/family#>.
@prefix math: <http://www.w3.org/2000/10/swap/math#>.

# --------------------------
# Data (a small family tree)
# --------------------------

:Adam :parentOf :Bob, :Carol .
:Bob :parentOf :Dave, :Eve .
:Carol :parentOf :Frank, :Grace .
:Dave :parentOf :Heidi .
:Eve :parentOf :Ivan .
:Frank :parentOf :Judy .

# Seed "branch" labels at generation 2 (so gen-1 siblings won't become cousins)
:Dave :branch :b .
:Eve :branch :b .
:Frank :branch :c .
:Grace :branch :c .

# Declare branch difference (so we can avoid any inequality built-in)
:b :differentFrom :c .
:c :differentFrom :b .

# -----------------------------------
# Rules (generation, branch, cousins)
# -----------------------------------

# Root generation
{ } => { :Adam :generation 0 } .

# Generation propagation: child.gen = parent.gen + 1
{
 ?P :parentOf ?C .
 ?P :generation ?G .
 (?G 1) math:sum ?G1 .
} => {
 ?C :generation ?G1 .
} .

# Branch propagation: child.branch = parent.branch
{
 ?P :parentOf ?C .
 ?P :branch ?B .
} => {
 ?C :branch ?B .
} .

# Cousins: same generation, different branch
{
 ?X :generation ?G .
 ?Y :generation ?G .
 ?X :branch ?BX .
 ?Y :branch ?BY .
 ?BX :differentFrom ?BY .
} => {
 ?X :cousin ?Y .
} .