SWI-Prolog -- indent

Documentation
- Reference manual
  - The SWI-Prolog library
    - library(strings): String utilities
- Packages

Availability::- use_module(library(strings)).(can be autoloaded)

[det]indent_lines(+Prefix, +In, -Out)

Add Prefix to the beginning of lines in In. Lines are separated by "\n" -- conversion to and from external forms (such as "\r\n") are typically done by the I/O predicates. Lines that consist entirely of whitespace are left as-is.