Multi-account credit cards
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Why not create multi-account credit cards? This will eliminate the need to carry a separate card for each account.
Such a card will allow a user to select the account to draw from (an active account will be indicated with a small blinking light on the credit card); it will be possible to swipe it just like a regular credit card. The card will also have the built-in battery which is good for 4 years of normal use.