Aliases: This file defines the main operations on aliases, a second name for a file.

Aliases are always textual constants (atoms)

Their first advantage is that they allow cleaning up code, by separating name from operation, eg YAP has a loop stream used to run the main top-level, which can be std0 originally but then changed to a pipe, a file, or a memory region Other important streams are the user streams Finally, the debugger uses debugger input and output

P redefined stream aliases are:

user: special alias, initially refers to all the three standard streams
user_input: initially refers to the stdandard input stream; +user_output: initially refers to the stdandard output stream;
user_error: initially refers to the stdandard error stream Often this is the same device asstderr`, just accessed in different ways
loop_stream: refers to the stream for the file or object being current consulted
debugger_input: refers to the stream used to send debugger commands, by default user_input
debugger_output: refers to the stream used to output debugging, by default user_error It must always be interactive