-#### check_string() requires two arguments, the first is a program and
-#### arguments, the second is its expected one-line output string. If
-#### the actual output does not match that string, an error message is
-#### printed and global variable "failed" is incremented.
-check_string() {
+#### run_test() requires two arguments, the first is a program and
+#### arguments, the second is its expected one-line output string.
+#### If the actual output does not match that string:
+#### an error message is printed and global variable "failed" is incremented;
+#### if there is an optional third argument, it is used in the error message.
+run_test() {