int	getbase( float gc, string sbase, string abase )
//  output a randdom base-pair sequence, with overall GC content of gc
{
	float	pgc;

	pgc = rand2();
	if( pgc < 0.5 * gc ){
		sbase = "c";		// C
		abase = "g";
	}else if( pgc >= 0.5 * gc && pgc < gc ){
		sbase = "g";		// G
		abase = "c";
	}else if( pgc >= gc && pgc < 0.5 + 0.5 * gc ){
		sbase = "a";		// A
		abase = "t";
	}else{
		sbase = "t";		// T
		abase = "a";
	}
};