Any reason this wouldn't work? "{cardhoursinpreviouslist} + {{%triggercardpreviouslistname}}"

[Geo]

I am attempting to create and automation that record the time a card spends in each list.  I have a custom field setup for each list.  The below regex works as any list so when the card is moved to any new list, the custom field corresponding to the previous list is updated with the time it was in that previous list.  However, if the card is moved back and forth it overwrites this time so I wanted to add to the existing value that is already in the custom field.  The below does not work.  If I replace this  {{%triggercardpreviouslistname}} with a static value or a custom field name, it does work.  

when a card is added to list "regex:/.*(\w+).*/", set custom field "{triggercardpreviouslistname}" to "{cardhoursinpreviouslist} + {{%triggercardpreviouslistname}}"

[Geo · Accepted solution]

Wait nevermind. I am new here!  One more {} to separate the variable worked!

when a card is added to list "regex:/.*(\w+).*/", set custom field "{triggercardpreviouslistname}" to "{cardhoursinpreviouslist} + {{%{triggercardpreviouslistname}}}"