Add grouping brackets

[fferflo]

Currently, there are no brackets that simply allow grouping subexpressions. Axis composition () can be used to group expressions

"(a b)... -> a... b..." # (a b) are grouped and can be repeated jointly using an ellipsis

which expands to

"(a0 b0) (a1 b1) -> a0 a1 b0 b1

but this is not always desirable. For example, the following related expression can currently not be expressed with ellipsis notation:

"a0 b0 a1 b1 -> a0 a1 b0 b1"

With grouping brackets of some kind (for example using {}), this could be written as

"{a b}... -> a... b...

Good:

• Allows writing some expressions more concisely.

Bad:

• Adds another type of bracket in addition to axis composition () and operation axes [], which adds to the complexity of einx expressions. This makes the notation more difficult to read/ learn and less intuitive.