The was no paper written on xgap as it was only a temporary
program. The program we were aimimg for includes many of the
operations performed by xgap but with an entirely new interface
and it is called gap. The reference is:
Bonfield,J K, Smith,K F and Staden,R. A New DNA Sequence Assembly Program.
Nucleic Acids Res. 23, 4992-4999 (1995)