The following conventions govern typing of formulas.
 
 (a)   Variables are  'p', 'q', 'r', 's'.
       Upper case will be converted to lower by the parser.
       Any other letter will be treated as garbage.
 
 (b)   Parentheses should be written in full, except that
 
         (i)    outermost parentheses are unnecessary
         (ii)   all right parentheses are unnecessary
         (iii)  a period (".") is equivalent to "("
 
       Because no scope rules are in force, association to 
       the left is assumed.  Therefore the right mate of 
       a left parenthesis immediately follows the shortest 
       intervening formula with dyadic main connective.
       For this reason right parentheses are redundant.

 (c)   Spaces are ignored.  RETURN terminates the formula.
 
 (d)   The connectives are "&" (ampersand), "v" (vee), 
       "~" (tilde) and ">" (greater than).  Implication
       may also be written as "->" (with optional minus).
       Sentential constants "t" and "f" are also allowed.