A customer macro: a fixed value, or a callback invoked with no arguments
right before the URL is built. "", null, and a callback result of
undefined omit parameters that use the macro. An undefined map entry is
not registered; in an update it unregisters the name.
A customer macro: a fixed value, or a callback invoked with no arguments right before the URL is built.
"",null, and a callback result ofundefinedomit parameters that use the macro. An undefined map entry is not registered; in an update it unregisters the name.