Simul efun: a_or_an - find the correct indefinite article

Synopsis:
	string a_or_an(string str)

Description:
	Is useful to determine whether a description has to start with
	an 'a' or 'an'.

Return value:
	returns "a" or "an" depending on first letter in <str>.

Examples:
	a_or_an("building") returns "a"
	a_or_an("emerald")  returns "an"

See also: efun/lower_case, efun/upper_case