key membership in multi-dimensional assoc. array

[Paul via Digitalmars-d-learn]

I found I can check for key membership in a multi-D aa...
```d
byte zKey = someval;
byte[byte][byte][byte] cubelist;

foreach(byte xK, yzcubelist; cubelist) {
  foreach(byte yK, zcubelist; yzcubelist) {
    foreach(byte zK, val; zcubelist) {
```
with this expression...
```d
    if(zKey in cubelist[xK][yK])
```
Is there a way to check for membership in the x & y dimensions?
```d
    if(yKey in cubelist[xK] ??? [zK])
```
*Thanks in advance for any ideas or solutions.*