Functions with optional first parameter

A small number of functions have an optional first parameter, where if it  
is absent, the context item is used:

boolean  is-card-number(string?)
boolean  valid(item*?)
boolean  relevant(item*?)
boolean  readonly(item*?)
boolean  required(item*?)

Is there reasoning behind this, rather than requiring (valid(.), for  


Received on Wednesday, 5 February 2020 11:55:38 UTC