Name: clear_bit - clear a bit in a string

Syntax:
	string clear_bit(string str, int num)

Description:
	The clear bit efun clears bit number num in string str.
	
Return value:
	It returns a new string where bit 'n' is cleared in string 'str'.
	Note that the old string 'str' is not modified.

Example:
	string s="C";
	s=set_bit(s, 1);

	This will do the following:
		C (ascii 67):      1000011
		clear_bit 1        -----0-
		A (ascii 65):      1000001

See also: efun/set_bit, efun/test_bit