prog 1 Sections StandardLIB Doc
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Def EndSwitch(x) == !undefined

is mentioned by

Def enum1_switch(value;el1;el2;el3)
Def == Switch(value)
Def == Case enum1_el1 => el1
Def == Case enum1_el2 => el2
Def == Case enum1_el3 => el3
Def == EndSwitch
[enum1_switch]

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prog 1 Sections StandardLIB Doc