head_a, head_b = [1, 2], [3, 4]
joined = head_a + head_b
# Rows are input features; columns are output features.
W = [[1, 0, 1, 0], [0, 1, 0, 1],
     [1, 0, -1, 0], [0, 1, 0, -1]]
output = [sum(joined[i]*W[i][j] for i in range(4)) for j in range(4)]
print("Concatenated:", joined)
print("Projected:", output)