Add LinguisticVariable and new method

Sources/FuzzyLogic/LinguisticVariable.swift

@@ -0,0 +1,39 @@
+import FuzzySets
+
+public struct LinguisticVariable<Universe, TermName: Hashable, Term: FuzzySetOperations, TermModifierName: Hashable> {
+    
+    public typealias TermModifier = (Grade) -> Grade
+    
+    let baseTerms: [TermName: Term]
+    let termModifiers: [TermModifierName: TermModifier]
+    
+    public init(
+        baseTerms: [TermName: Term],
+        termModifiers: [TermModifierName: TermModifier] = [:]
+    ) {
+        self.baseTerms = baseTerms
+        self.termModifiers = termModifiers
+    }
+    
+    public func term(_ name: TermName) -> Term? {
+        baseTerms[name]
+    }
+    
+    public subscript(_ termName: TermName) -> Term? {
+        term(termName)
+    }
+    
+    public subscript(_ termName: TermName, applyModifier modifierName: TermModifierName) -> Term? {
+        applyModifier(modifierName, to: termName)
+    }
+    
+    public func applyModifier(_ modifierName: TermModifierName, to termName: TermName) -> Term? {
+        guard let term = term(termName) else { return nil }
+        return applyModifier(modifierName, to: term)
+    }
+    
+    public func applyModifier(_ modifierName: TermModifierName, to term: Term) -> Term? {
+        guard let modifier = termModifiers[modifierName] else { return nil }
+        return term.appliedCustomFunction(modifier)
+    }
+}

Sources/FuzzySets/FuzzySetOperations.swift

@@ -13,4 +13,6 @@ public protocol FuzzySetOperations: FuzzySet {
     func symmetricDifference(_ other: Self, method: SymmetricDifferenceFunction) -> Self
     
     func power(_ n: Double) -> Self
+    
+    func appliedCustomFunction(_ function: @escaping (Grade) -> Grade) -> Self
 }

Sources/FuzzySets/Implementations/AnyFuzzySet.swift

@@ -48,7 +48,11 @@ extension AnyFuzzySet: FuzzySetOperations {
     }
     
     public func power(_ n: Double) -> Self {
-        return .init { Double.pow(membershipFunction($0), n) }
+        .init { Double.pow(membershipFunction($0), n) }
+    }
+    
+    public func appliedCustomFunction(_ function: @escaping (Grade) -> Grade) -> Self {
+        .init { function(membershipFunction($0)) }
     }
 }
 

Sources/FuzzySets/Implementations/DiscreteMutableFuzzySet.swift

@@ -62,6 +62,12 @@ extension DiscreteMutableFuzzySet: FuzzySetOperations {
         result.applyPower(n)
         return result
     }
+    
+    public func appliedCustomFunction(_ function: @escaping (Grade) -> Grade) -> Self {
+        var result = self
+        result.applyFunction(function)
+        return result
+    }
 }
 
 // MARK: - Mutability

Sources/FuzzySets/Implementations/IterableFuzzySet.swift

@@ -91,6 +91,12 @@ extension IterableFuzzySet: FuzzySetOperations {
             Double.pow(function($0), n)
         }
     }
+    
+    public func appliedCustomFunction(_ function: @escaping (Grade) -> Grade) -> IterableFuzzySet<Universe, S> {
+        .init(sequence) {
+            function(self.function($0))
+        }
+    }
 }
 
 // MARK: - Sequence