Clarify meaning of a[i] += 1

[rakudrama]

The language spec 0.06 grammar permits

   a[i] += 1;

but section 10.16.1 does not describe a behaviour.

I would like to see the expression be legal, otherwise many numeric codes will look strained in Dart.

I suggest the meaning by analogy with e1.v op= e2

e1[e2] op= e3
-->
((a, i) => a[i] = (a[i] op e3))(e1, e2) // a, i fresh variables.

Footnote:
A consequence of permitting a[i] += 1 is that a[i]++ is now defined.

To prevent the following from being strange, I suggest that the value of a[i]=e is defined consistently with the value of a.v=e

var v1 = a[i];
var v2 = a[i]++;
if (v1 + 1 != v2) { strange(); }

[gbracha]

Set owner to @gbracha.
Added Accepted label.

[gbracha]

Clarified.

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Added Done label.