From Tapestries MUCK
This is a simple conditional command. It evaluates the 'check' argument and if it is true, then it evaluates the 'true' argument and returns it's result.
If 'check' does not evaluate as true, then it will evaluate the 'false' argument, if there is one, and returns it's result.
If there is no false argument, and 'check' evaluated false, then it returns a null string.