Multivalue computed field

Hello,
I want to handle multiple values for the same computed field.
In computed field's GUI I read "For multi-value computed fields continue with $entity_field[1]['value']" but how can I take this value (and so on) in display code? A print_r of $entity_field_item shows only one value, while $entity_field_multi, $entity_field_multi_item (and other attempts) all result in "undefined variable".
Moreover, how can I take the multiple values in other places? Few words to explain this question: let's say I have a computed field called "field_a". Suppose that in computed code I set $entity_field[0]['value'], $entity_field[1]['value'] and $entity_field[2]['value']. Now let's say I write a second computed field (call it field_b) where I need second and third values from field_a. How can I do this?
Thanks in advance