/* Propositional logic parser
   Sten Morten Andersen, 2003 
   
Grammar:

formula    ::= expression { ('>' | '=') formula }
expression ::= term { 'v' term }
term       ::= factor { '&' factor }
factor     ::= 'a' | 'b' | 'c' | .. | 'z' | 
               '-' formula |
               '(' formula ')'
*/
char ch;
void formula();

void error(char *s) { printf("%s. Seen %c\n", s, ch); exit(1); }

void get_char() {
	do ch = getchar(); while ( ch <= ' ' );
}

void factor() {
	if ( ch >= 'a' && ch <= 'z' ) ch = 'L';
	switch( ch ) {
		case 'L': get_char(); break;
		case '-':
			get_char();
			formula();
		break;
		case '(':
			get_char();
			formula();
			if ( ch != ')' ) error("Missing )");
			get_char();
		break;
		default:
			error("Expected a..z or (formula)");
	}	 
}


void term() {
	factor();
	while ( ch == '&' ) {
		get_char();
		factor();
	} 
}

void expression() {
	term();
	while ( ch == 'v' ) {
		get_char();
		term();
	} 
}

void formula() {
	expression();
	while ( ch == '>' || ch == '=') {
		get_char();
		formula();
	} 
}

int main() {
	get_char();
	formula();
	if ( ch != '.') error ("Expected .");
	return 0;
}