Example:

Switch Stack A:
Switch 1
power supply 1
Switch 2
power supply 1
Switch 3
power supply 1
power supply 2

• • • The number of switches can vary from stack to stack, and the number of power supplies can vary from switch to switch.

I wrote a LLD discovery rule to discovery all of the members of the stack:
name: stack unit
key: stackunit
snmp oid: discovery[{#STACKUNIT},.1.3.6.1.4.1.6027.3.10.1.2.2.1.2]
I also created a macro: {$STACKUNIT} — no value assigned.

Now that I can detect the stack units, I want to detect the power supplies in each unit:

name: powersupplies
key: powersupply
snmp oid: discovery[{#POWERSUPPLY},.1.3.6.1.4.1.6027.3.10.1.2.3.1.3]

Next I create an Item prototype to monitor the power supplies:

Name: Unit {#STACKUNIT} - Power Supply {#POWERSUPPLY} - Status:
Key: pwrsupply-["{#STACKUNIT}-{#POWERSUPPLY}"]
Snmp OID: .1.3.6.1.4.1.6027.3.10.1.2.3.1.2.{#STACKUNIT}.{#POWERSUPPLY}

It is not working, and I am getting this:

365:20150805:163334.350 item "pssw00:pwrsupply-["{#STACKUNIT}-1"]" became not supported: snmp_parse_oid(): cannot parse OID ".1.3.6.1.4.1.6027.3.10.1.2.3.1.2.{#STACKUNIT}.1".

It doesn't resolve {#STACKUNIT} for some reason.