What's happening here is that we are checking to make sure that the other
lister we're using has not got a custom handler and returning the result to
whoever called us.
1checkhandler:
2 lister query otherhandle handler
3 return ~(result = 'RESULT' | result = '')
Line:
1 The sub-routine label.
2 We ask for the handler associated with the other lister.
3 OK, if result = 'RESULT' or an empty string then the statement inside
the brackets is true (1), but we are returning the opposite, hence
the ~ negation sign outside the brackets. So if there is no handler
associated with the lister we will return false (0).
If result did equal something other than RESULT and an empty string,
then the statement inside would be false (0) but we would return
true (1) because of the ~ outside.
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