add up the columns into wide, overlapping limbs. // r0 = a0×b0 + 19×(a1×b4 + a2×b3 + a3×b2 + a4×b1) r0 := mul(a0, b0) r0 = addMul19(r0, a1, b4) r0 = addMul19(r0, a2, b3) r0 = addMul19(r0, a3, b2) r0 = addMul19(r0, a4, b1) // r1 = a0×b1 + a1×b0 + 19×(a2×b4 + a3×b3 + a4×b2) r1 := mul(a0, b1) r1 = addMul(r1, a1, b0) r1 = addMul19(r1, a2, b4) r1 = addMul19(r1, a3, b3) r1 = addMul19(r1, a4, b2) // r2 = a0×b2 + a1×b1 + a2×b0 + 19×(a3×b4 + a4×b3) r2 := mul(a0, b2) r2 = addMul(r2, a1, b1) r2 = addMul(r2, a2,...

	A2 = A1
)

type (
	B1 string
	B2 B1
	B3 []B1
	B4 B3
)

func f[P any](x P) { … }
</pre>

<p>
The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
and <code>B2</code> is <code>string</code>.
The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.