A nested sequence containing a single unit is redundant

[lgarron]

The following prints false:

var original = new Alg.Group(
  new Alg.Sequence([
    new Alg.BlockMove("R", 1)
  ]),
  1
)
var parsed = parse(algToString(original));
console.log(structureEquals(original, parsed));

Ideally, I'd like there to be only a single representation of this alg.
One possibility is to have a nested kind of sequence that must have at least two units in it. The parser actually uses this requirement to avoid unnecessary nesting (which is why parsed is different):

alg.js/src/jison_parser/index.jison

Line 147 in 716c91d

: REPEATED_UNIT OPTIONAL_WHITESPACE REPEATED_UNIT

Ideas:

1. Create a different kind of sequence for a nested alg that requires at least two members.

• Pro: The top-level alg class could also handle the empty alg (Can't keep class names using uglifyjs-webpack-plugin with "mode": "production" #5).
• Con: Nesting certain (or all) top-level algs inside another alg would not be possible without changing the top-level representation into an equivalent nested representation.

2. Define every Algorithm with nested algs to have its own lists, e.g. a commutator has {A: Unit[], B: Unit[]}. This requires making sure to call the Sequence processing code for each of the components in every relevant Algorithm in every traversal, possibly leading to subtle bugs.
3. Don't allow anything except a Sequence to contain a Unit, and require a Sequence layer.

• This results in larger data structures, and an extra level of access in some cases. However, code might be more consistent, because looking inside a nested alg always requires going through a Sequence the same way.

See https://gist.github.com/lgarron/f33d88a571f4bc9536801a8a7b035a91 for more thoughts on this.