Is

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The reserved word is appears as an

operator

The operator is tests, whether an object (first operand) is an instance of a class or its children.

fruit is citrusFruit

is equivalent to

fruit.inheritsFrom(citrusFruit)

.

The expression is true if and only if fruit descends from citrusFruit.

modifier

If {$modeSwitch nestedProcVars+} or {$mode ISO} routine variables declared with the modifier is nested allows those to be assigned to nested routines. Otherwise only global routines' addresses can be assigned to routine variables.