isbound and isunboundThe filters
isunboundcheck whether a variable is bound (has a value) or not. Each filter is followed by a variable name.
isbound Foo isunbound Bar
isbound filter is true if its argument is the name of a bound variable.
isunbound filter is true if its argument is not the name of a bound variable.
These filters can be used so that variable values can be overriden from the command line, particularly in conjunction with the -assign command line option.
Sometimes they can be useful as boolean flags as well.