function str2enum( ctype ) {
	var v = ENUM[ ctype ];
	return ( typeof v === 'number' ) ? v : -1; // note: we include this guard to prevent walking the prototype chain
}
Returns the enumeration constant associated with a function continuity type string.

@private
@param {string} ctype - function continuity type string
@returns {(integer|null)} integer value or null

@example
var v = str2enum( 'left-continuous' );
// returns <number>