/**
Turns a digit into its English name
@param n an integer between 1 and 9
@return the name of n ("one" ... "nine")
*/
string digit_name(int n);
/**
Turns a number between 10 and 19 into its English name.
@param n an integer between 10 and 19
@return the name of n ("ten"..."nineteen")
*/
string teen_name(int n);
/**
Gives the English name of a multiple of 10
@param n an integer between 2 and 9
@return the name of 10 * n ("twenty"..."ninety")
*/
string tens_name(int n);