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→{{header|Ada}}: Fixed typo (ect. → etc.)
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-- non-destructively search for old values
function Peek(V: Variable; Generation: Natural := 1) return Item_Type;
-- V.Peek(0) returns current value; V.Peek(1) the previous value,
-- when calling V.Peek(i), i must be in 0 .. V.Defined-1, else Constraint_Error is raised
-- destructively restore previous value
procedure Undo(V: in out Variable);
-- old V.Peek(0) is forgotten, old V.Peek(i) is new V.Peek(i-1),
-- accordingly, V.Defined decrements by 1
-- special case: if V.Defined=0 then V.Undo does not change V
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