An 18-month-old Missouri law that allows casinos to issue credit to well-heeled patrons is working as intended, casino operators say, but a few tweaks could make it even more useful. Under provisions that went into effect in July 2014, seven of the state’s 13 casinos now offer lines of credit to customers who qualify to borrow at least $10,000 and have the ability to pay it back within 30 days.