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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.