Term Rewriting

General

We use an adaption of the ARI format, so TRSs are represented as S-Expressions (see here, Sec. 3.1). Our format differs from the format used at COCO as follows:

• We do not impose any variable conditions, so a rewrite rule is a pair of arbitrary terms.
• Identifiers must be valid SMT-LIB symbols (see here, Sec. 3.2), whereas COCO uses a more liberal definition. The following is actively discussed and might be changed: Both simple and quoted symbols are allowed. To ease parsing, we impose the following additional restrictions:
  • Quoted identifiers must not contain whitespace, parantheses, or semicolons.
  • All occurrences of the same identifier must be either quoted or unquoted. So

    (rule |a| |a|)

    is valid, but

    (rule a |a|)

    is invalid.
• Rules are annotated with optional costs, which are natural numbers. This allows, e.g., to model relative rewriting (by setting the cost of relative rules to 0). Categories may disallow costs. So rules are defined as follows:

rule ::= ( rule term term cost? )
cond ::= :cost number

In contrast to the former XTC format, the goal of the analysis is implicitly specified by the category.

Termination

All termination categories consider termination w.r.t. arbitrary start terms.

Relative Termination

All categories for relative termination consider full rewriting. See here for the format of all categories for relative termination.

TRS Relative

• no further restrictions

SRS Relative

• just unary function symbols

Non-Relative Termination

All categories for non-relative termination disallow costs.

TRS Standard

• full rewriting
• see here

SRS Standard

• full rewriting
• just unary function symbols
• see here

TRS Contextsensitive

• context-sensitive rewriting
• see here

TRS Equational

Here, a theory may be added to declarations of function symbols.

fun ::= ( fun identifier number theory? )
theory ::= :theory [A | C | AC]

• full rewriting modulo associativity / commutativity / associativity and commutativity

TRS Innermost

• innermost rewriting
• see here

TRS Outermost

• outermost rewriting
• see here

TRS Conditional

• full conditional rewriting
• see here
• currently, we only support the condition-type oriented

TRS Conditional - Operational Termination

• TODO clarify the difference to TRS Conditional
• see here
• currently, we only support the condition-type oriented

Complexity

See here for all complexity categories.

Runtime Complexity

All categories for runtime complexity consider basic start terms only.

Runtime Complexity Innermost

• innermost rewriting

Runtime Complexity Full

• full rewriting

Derivational Complexity

All categories for derivational complexity consider arbitrary start terms.

Derivational Complexity Innermost

• innermost rewriting

Derivational Complexity Full

• full rewriting