Orbital library

## orbital.logic.trs Interface Substitution

All Superinterfaces:
Function, Functor

`public interface Substitutionextends Function`

Term substitution function.

 A (uniform) substitution σ is a total endomorphism σ:Term(Σ)→Term(Σ) with m σ(f(t1,…,tn)) = σ(f)(σ(t1),…,σ(tn)) ∀f(t1,…,tn)∈Term(Σ) fin σ|V = id p.t. (⇔ supp(σ) := {x∈V ¦ σ(x)≠x} is finite) Note: Substitutions are usually restricted to "proper" variable substitutions that only substitute variables, i.e. σ(f)=f for functions and predicates. Often, variable substitutions are even restricted to admissible variable substitutions that only substitute free variables, such that it does not lead to collisions. Otherwise the application of a variable substitution would possibly introduce new (illegal) bindings inside the scope of a quantifier. A basic substitution (also see `Substitution.Matcher`), i.e. a mapping σ0:V→Term(Σ) with fin σ0 = id p.t. (⇔ supp(σ) := {x∈V ¦ σ(x)≠x} is finite) can be extended uniquely (UP) to a variable substitution σ:Term(Σ)→Term(Σ) such that σ|V = σ0.
"Substitutions are the homomorphic continuation of variable replacements."
Let σ be a substitution.
variable renaming
σ is a variable renaming :⇔ σ:V→V is injective ⇔ σ(V)⊆V ∧ σ|supp(σ) is injective
idempotent
σ is idempotent :⇔ σ ∘ σ = σ ⇔ supp(σ)∩σ(supp(σ))=∅ ⇔ no variable xi occurs in a tj.
If a variable substitution has a left-inverse variable substitution, then it is only a variable renaming. For variable substitutions that have a right-inverse variable substitution this is true if their supports are disjoint.

We denote a substitution σ of supp(σ)={x1,…,xn} replacing xi with ti by

σ = [x1→t1,x2→t2,…,xn→tn] = [t1/x1,t2/x2,…,tn/xn]
σ(t) is called an instance of the term t.

Substitutions are for computer science what permutations (finite symmetric group) are for mathematics.

Author:
André Platzer
`Substitutions`, `Substitutions.getInstance(Collection)`

Nested Class Summary
`static interface` `Substitution.Matcher`
Interface for matching and replacing elementary terms within a substitution.

Nested classes/interfaces inherited from interface orbital.logic.functor.Function
`Function.Composite`

Nested classes/interfaces inherited from interface orbital.logic.functor.Functor
`Functor.Specification`

Field Summary

Fields inherited from interface orbital.logic.functor.Function
`callTypeDeclaration`

Method Summary
` java.lang.Object` `apply(java.lang.Object term)`
Apply this substitution σ to term.
` java.util.Collection` `getReplacements()`
Get the set of elementary replacements.

Methods inherited from interface orbital.logic.functor.Functor
`equals, hashCode, toString`

Method Detail

### getReplacements

`java.util.Collection getReplacements()`
Get the set of elementary replacements.

For a substitution σ = [x1→t1,x2→t2,…,xn→tn] the set of elementary replacements is {x1→t1,x2→t2,…,xn→tn}. Those elementary replacements are each specified by an implementation of `Substitution.Matcher`.

Note that the return-type is not fixed to sets, but would just as well allow lists as implementations although the order is not relevant for variable substitutions.

Returns:
the list of matchers used for replacement in this substitution.

### apply

`java.lang.Object apply(java.lang.Object term)`
Apply this substitution σ to term.

A (uniform) substitution [x→t] replaces all occurrences of x with t. Whereas for substitutions with multiple replacement directions [x1→t1,x2→t2,… xn→tn], only the first applicable replacement will be applied on subterms.

Specified by:
`apply` in interface `Function`
Parameters:
`term` - the term object that will be decomposed according to `Composite` for applying the substitution, with variables identified via `Variable.isVariable()`. Substitutions automatically `map` themselves over `Collection` and arrays.
Returns:
σ(term)
Throws:
`java.lang.ArrayStoreException` - if this method tried to store a part of the result in an array, but the substitution list produced a replacement of an illegal type.
`java.lang.ClassCastException` - if the substitution list produced a replacement of an illegal type.
Preconditions:
"the `Composite`s occuring in term support `Composite.construct(Object,Object)` in order to allow being substituted by an object of equal class"

Orbital library
1.3.0: 11 Apr 2009