How Y-bricks are more complicated than H-bricks.

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The unit H-brick can be stacked vertically:

H1_S1_D0 stack of three

The same applies to the unit P-brick, P1_S1_D0.

On the other hand, the unit Y-brick cannot be so aligned, with six different ways to stack three bricks:

Y3_S1_D0

In fact, a tall stack can be arranged into a spiral:

Y3_S1_D0