FUNCTION query_YN,prompt
;
; Prints a prompt which requires a 'yes' or 'no' response.
; The prompt will be repeated if the first character is not Y or N
PRINT,prompt
answer = ' '
READ,answer
a = STRUPCASE(STRMID(STRTRIM(answer,2),0,1))
WHILE(a ne 'Y' AND a ne 'N') DO BEGIN
   PRINT, 'Please answer Yes or No'
   READ,answer
   a = STRUPCASE(STRMID(answer,0,1))
ENDWHILE
RETURN, a
end