The list of escape sequences is compatible with the ISO standard, but contains one extension and the interpretation of numerically specified characters is slightly more flexible to improve compatibility. All input characters upto but not including the first non-layout character are skipped. This allows for the specification of pretty-looking long lines. For compatibility with Quintus Prolog.
Skips input till the next non-layout character or to the end of the next line.
23 is just an example. The SWI-Prolog definition allows for ISO compatible specification, but is compatible with other implementations. The rules and remarks for hexadecimal specifications apply to octal specifications too, but the maximum allowed number of octal digits is 3.