[<key> IN MAH]* <var>This (l-value and r-value) indexes a (multidimensional, if more than one) array, and currently only positive integral keys are supported. While there is no operator precedence defined as of this recommendation, it is probably best for IN MAH to have very high precedence. If this is an l-value (left hand side of an assignment) the array should be expanded to be at least <key> elements in that direction.
a IN MAH b IN MAH c IN MAH var
Would be equivalent to
var[c][b][a]
in C, and would expand each “axis” of the array as necessary to fit that element in. If it is an r-value (that is, being accessed instead of assigned) and the index is out of the current bounds of the array, an error should be generated.
If the var is accessed without an index, it should be operated upon for each element, in most cases. See visible for a brief discussion of this.
In a future recommendation, there will hopefully be a mechanism for getting the bounds of an array without having to keep track.
See also: lol r