add more linear-x examples

examples/linear-15.spec

+Workflow
+
+forallmay i,j O(i,j) -> A += (i,j)
+forall i,j A(i,j) -> B += (i,j)
+forall i,j B(i,j) -> C += (i,j)
+forall i,j C(i,j) -> D += (i,j)
+forall i,j D(i,j) -> E += (i,j)
+forall i,j E(i,j) -> F += (i,j)
+forall i,j F(i,j) -> G += (i,j)
+forall i,j G(i,j) -> H += (i,j)
+forall i,j H(i,j) -> I += (i,j)
+forall i,j I(i,j) -> J += (i,j)
+forall i,j J(i,j) -> K += (i,j)
+forall i,j K(i,j) -> L += (i,j)
+forall i,j L(i,j) -> M += (i,j)
+forall i,j M(i,j) -> N += (i,j)
+forall i,j N(i,j) -> O += (i,j)
+
+Target
+
+O(x,y)

examples/linear20.spec

+Workflow
+
+forallmay i,j O(i,j) -> A += (i,j)
+forall i,j A(i,j) -> B += (i,j)
+forall i,j B(i,j) -> C += (i,j)
+forall i,j C(i,j) -> D += (i,j)
+forall i,j D(i,j) -> E += (i,j)
+forall i,j E(i,j) -> F += (i,j)
+forall i,j F(i,j) -> G += (i,j)
+forall i,j G(i,j) -> H += (i,j)
+forall i,j H(i,j) -> I += (i,j)
+forall i,j I(i,j) -> J += (i,j)
+forall i,j J(i,j) -> K += (i,j)
+forall i,j K(i,j) -> L += (i,j)
+forall i,j L(i,j) -> M += (i,j)
+forall i,j M(i,j) -> N += (i,j)
+forall i,j N(i,j) -> O += (i,j)
+forall i,j O(i,j) -> P += (i,j)
+forall i,j P(i,j) -> Q += (i,j)
+forall i,j Q(i,j) -> R += (i,j)
+forall i,j R(i,j) -> S += (i,j)
+forall i,j S(i,j) -> T += (i,j)
+
+Target
+
+T(x,y)