isbound and isunbound

The filters isbound and isunbound check whether a variable is bound (has a value) or not. Each filter is followed by a variable name.

    isbound Foo
    isunbound Bar

The isbound filter is true if its argument is the name of a bound variable.

The 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.