// for validation during parsing.
func (d *dataTable[EI, E]) insert(id EI, data E) error {
	if d.sparse == nil {
		d.sparse = make(map[EI]E)
	}
	if existing, ok := d.get(id); ok {
		return fmt.Errorf("multiple %Ts with the same ID: id=%d, new=%v, existing=%v", data, id, data, existing)
	}
	d.sparse[id] = data
	return nil
}

// compactify attempts to compact sparse into dense.
//

00000130  92 42 5b c2 81 c0 be 97  79 98 40 fb 4f 6d 14 fd  |.B[.....y.@.Om..|
00000140  2b 13 8b c2 a5 2e 67 d8  d4 09 9e d6 22 38 b7 4a  |+.....g....."8.J|
00000150  0b 74 73 2b c2 34 f1 d1  93 e5 96 d9 74 7b f3 58  |.ts+.4......t{.X|
00000160  9f 6c 61 3c c0 b0 41 d4  d9 2b 2b 24 23 77 5b 1c  |.la<..A..++$#w[.|
00000170  3b bd 75 5d ce 20 54 cf  a1 63 87 1d 1e 24 c4 f3  |;.u]. T..c...$..|

00000130  92 42 5b c2 81 c0 be 97  79 98 40 fb 4f 6d 14 fd  |.B[.....y.@.Om..|
00000140  2b 13 8b c2 a5 2e 67 d8  d4 09 9e d6 22 38 b7 4a  |+.....g....."8.J|
00000150  0b 74 73 2b c2 34 f1 d1  93 e5 96 d9 74 7b f3 58  |.ts+.4......t{.X|
00000160  9f 6c 61 3c c0 b0 41 d4  d9 2b 2b 24 23 77 5b 1c  |.la<..A..++$#w[.|
00000170  3b bd 75 5d ce 20 54 cf  a1 63 87 1d 1e 24 c4 f3  |;.u]. T..c...$..|

00000290  a2 5d fe 1e 52 45 88 7a  36 47 a5 08 0d 92 42 5b  |.]..RE.z6G....B[|
000002a0  c2 81 c0 be 97 79 98 40  fb 4f 6d 14 fd 2b 13 8b  |.....y.@.Om..+..|
000002b0  c2 a5 2e 67 d8 d4 09 9e  d6 22 38 b7 4a 0b 74 73  |...g....."8.J.ts|
000002c0  2b c2 34 f1 d1 93 e5 96  d9 74 7b f3 58 9f 6c 61  |+.4......t{.X.la|
000002d0  3c c0 b0 41 d4 d9 2b 2b  24 23 77 5b 1c 3b bd 75  |<..A..++$#w[.;.u|