private static function pfx( $tokens, $vars = array() ) {
			if ( false == $tokens ) {
				return false;
			}
			$stack = new WC_Eval_Math_Stack;

			foreach ( $tokens as $token ) { // nice and easy
				// if the token is a binary operator, pop two values off the stack, do the operation, and push the result back on
				if ( in_array( $token, array( '+', '-', '*', '/', '^' ) ) ) {
					if ( is_null( $op2 = $stack->pop() ) ) {
						return self::trigger( "internal error" );
					}
					if ( is_null( $op1 = $stack->pop() ) ) {
						return self::trigger( "internal error" );
					}
					switch ( $token ) {
						case '+':
							$stack->push( $op1 + $op2 );
							break;
						case '-':
							$stack->push( $op1 - $op2 );
							break;
						case '*':
							$stack->push( $op1 * $op2 );
							break;
						case '/':
							if ( 0 == $op2 ) {
								return self::trigger( 'division by zero' );
							}
							$stack->push( $op1 / $op2 );
							break;
						case '^':
							$stack->push( pow( $op1, $op2 ) );
							break;
					}
					// if the token is a unary operator, pop one value off the stack, do the operation, and push it back on
				} elseif ( '_' === $token ) {
					$stack->push( -1 * $stack->pop() );
					// if the token is a function, pop arguments off the stack, hand them to the function, and push the result back on
				} elseif ( ! preg_match( "/^([a-z]\w*)\($/", $token, $matches ) ) {
					if ( is_numeric( $token ) ) {
						$stack->push( $token );
					} elseif ( array_key_exists( $token, self::$v ) ) {
						$stack->push( self::$v[ $token ] );
					} elseif ( array_key_exists( $token, $vars ) ) {
						$stack->push( $vars[ $token ] );
					} else {
						return self::trigger( "undefined variable '$token'" );
					}
				}
			}
			// when we're out of tokens, the stack should have a single element, the final result
			if ( 1 != $stack->count ) {
				return self::trigger( "internal error" );
			}
			return $stack->pop();
		}