PGASetUserFunction(8) on Linux

The text 1d451951776f, as Debian 8 (Jessie), Debian 7 (Wheezy), Debian 6.0 (Squeeze), Debian 5.0 (Lenny) shipped it. The page shows this text now.

specifies the name of a user-written function call to provide a specific GA capability (e.g., crossover, mutation, etc.).