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Pascal has a built-in set type — a compact, type-safe bitset over a small ordinal range.

type
    TDay = (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
    TDays = set of TDay;

var
    weekdays: TDays;
begin
    weekdays := [Mon, Tue, Wed, Thu, Fri];

    if Mon in weekdays then
        WriteLn('yes');

    weekdays := weekdays + [Sat];      // add
    weekdays := weekdays - [Mon];      // remove
end.

Set operations:

• + — union
• - — difference
• * — intersection
• <= — subset
• >= — superset
• = — equal
• in — membership

Sets work on any ordinal type with a small range:

var
    digits: set of '0'..'9';
    primes: set of 1..255;
begin
    digits := ['0', '1', '2', '3'];
    primes := [2, 3, 5, 7, 11, 13];
end.

Pascal sets are bit fields under the hood — extremely fast membership tests. The downside: range can't exceed 256 elements (in standard FPC).

Use cases:

• Character class checks: if c in ['a'..'z', 'A'..'Z'] then
• State flags: if optActive in opts then
• Permission masks

For larger sets (millions of elements), use a TBitSet class or hash-based set from a library.