QSET
QSET is an abandoned programming language designed by User:Peter in 2022. All data consists of sets of numbers, which may or may not be structured in ordered pairs. It's named after the set of rational numbers, which contains every value that can be used in QSET.
Language Overview
There are three different data types in QSET: Sets, ordered pairs and functions. Sets can contain up to (countably) infinite values, and any operator that changes a set changes each value independently. Numerical literals are treated as sets with only one element. Ordered pairs can include two sets, a set and an ordered pair or two ordered pairs. Functions can take in any amount of sets and/or pairs as parameters, and will return another set or pair without side effects.
Commands
Instruction  Parameters  Description  Example usage 

a = b

Identifier, set/pair  Sets a , which is a set or a pair, equal to b , which must have the same data type as a .

a = 5

[a, b]

Literal, literal  Any number between and including a and b is in this set. Replace a with < to include all numbers lower than b, or b with > to include all numbers greater than a.

[3, >]

{a, b}

Literal, literal  Any number between but not including a and b is in this set. You can use < and > for the same purposes as with the [a, b] , and you can use curly brackets and square brackets for the same set.

{5, 9]

Number  Literal  A number outside of curly or square brackets counts as a set containing only one element.  a = 5

a,b

Set/pair, set/pair  An ordered pair containing the values a and b , which can be sets or pairs.

a = (5, [2, 6}), 3

<a

Pair  The left side of pair a .

a = <b

a>

Pair  The right side of pair a .

a> = b

a: b

Identifier, data type  Declares a as a variable of type b (Set /Pair ). May be used in the same line as a = .

a: Set = {<, 8]

a :: b = c

Identifier, data type, identifier (constant) / literal  Declares a as a constant variable of type b (Set /Pair ) with value c .

a :: Pair = 5, 7

a & b

Set, set  Returns the intersection of a and b .

a = b & [3, 9]

b  Set, set  Returns the union of a and b .

{12, 100] 
a ^ b

Set, set  Returns all values in either a or b but not both.

a = a ^ b

!a

Set  Returns the complement of a .

a = !1

a+b

Set  Returns every combination of any value in a added to any value in b .

a = {1, 5] + [3, 2}

a*b

Set  Returns every combination of any value in a multiplied by any value in b .

a = b * 0.5

Natural

None  A builtin set of all natural numbers, not including 0.  a = Natural & {<, 10]

Integer

None  A builtin set of all integers.  a = !Integer

Rational

None  A builtin set of all rational numbers.  a = Rational

[] or {}

None  An empty set.  Equal :: a: Set, b: Set > Set = a^b ? {}, a

a?b

Set, pair  Returns the right side of b if a is empty, and returns the left side otherwise.

a & [0, >} ? 1, 1 
a :: b > c = d

Identifier, identifier declarations, data type, function  A pure function taking in argument(s) b and returning d , which has type c .

Sum :: a: Set, b: Set > Set = a + b 
a(b)

Function, argument(s)  Calls function a , using argument(s) b , separated by commas.

Sum(a, 1) 


None  The rest of the line is a comment.  a = b  make a equal b

import a

File name  Import a (without an extension or quotation marks) so functions from the file can be used in this one.

import simple maths

public a

Identifier declaration  Gives other files access to a if they have imported this one.

public Sum :: a: Pair > Set = <a + a>

Example programs
Here are some simple QSET examples.
FizzBuzz
QSET doesn't have IO or strings, so FizzBuzz is simply implemented as a function returning 1, 2, 3 or a natural number depending on the value used.
Fizz :: n: Set > Set = n & (Integer * 3) ? 1, {}  Integer * 3 = 3, 6, 9, 12, 15 ... Buzz :: n: Set > Set = n & (Integer * 5) ? 2, {} public FizzBuzz :: n: Set > Set = (Fizz(n) + Buzz(n)) ? (Fizz(n) + Buzz(n)), n
Even or odd?
IsEven :: n: Set > Set = n & (Integer * 2) ? n : {}
Fibonacci
Fibonacci :: n: Set > Set = n & 1 ? 1 n & [<, 1} ? 0