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(elisp)Conditional Display

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Conditional Display Specifications
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   You can make any display specification conditional.  To do that,
package it in another list of the form `(when CONDITION .  SPEC)'.
Then the specification SPEC applies only when CONDITION evaluates to a
non-`nil' value.  During the evaluation, `object' is bound to the
string or buffer having the conditional `display' property.  `position'
and `buffer-position' are bound to the position within `object' and the
buffer position where the `display' property was found, respectively.
Both positions can be different when `object' is a string.