#!/usr/local/bin/cadabra2
{m,n,p,q,r,s,t,u,v,w,a,b,c,d,e,f}::Indices(vector).
W_{m n p q}::WeylTensor.
ex:= W_{p q r s} W_{p t r u} W_{t v q w} W_{u v s w}
   - W_{p q r s} W_{p q t u} W_{r v t w} W_{s v u w}
   - W_{m n a b} W_{n p b c} W_{m s c d} W_{s p d a}
   + (1/4) W_{m n a b} W_{p s b a} W_{m p c d} W_{n s d c};
young_project_tensor(_, modulo_monoterm=True);
distribute(_)
canonicalise(_)
rename_dummies(_);