# Documentation

Mathlib.Tactic.Linarith.Datatypes

# Datatypes for linarith#

Some of the data structures here are used in multiple parts of the tactic. We split them into their own file.

This file also contains a few convenient auxiliary functions.

def Linarith.linarithTraceProofs {α : Type} [inst : ] (s : α) (l : ) :

A shorthand for tracing the types of a list of proof terms when the trace.linarith option is set to true.

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### Linear expressions #

A linear expression is a list of pairs of variable indices and coefficients, representing the sum of the products of each coefficient with its corresponding variable.

Some functions on Linexp assume that n : Nat occurs at most once as the first element of a pair, and that the list is sorted in decreasing order of the first argument. This is not enforced by the type but the operations here preserve it.

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Add two Linexps together componentwise. Preserves sorting and uniqueness of the first argument.

l.scale c scales the values in l by c without modifying the order or keys.

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• = if c = 0 then [] else if c = 1 then l else List.map (fun x => match x with | (n, z) => (n, z * c)) l

l.get n returns the value in l associated with key n, if it exists, and none otherwise. This function assumes that l is sorted in decreasing order of the first argument, that is, it will return none as soon as it finds a key smaller than n.

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l.contains n is true iff n is the first element of a pair in l.

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• = Option.isSome

l.zfind n returns the value associated with key n if there is one, and 0 otherwise.

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• = match with | none => 0 | some v => v

l.vars returns the list of variables that occur in l.

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Defines a lex ordering on Linexp. This function is performance critical.

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### Inequalities #

inductive Linarith.Ineq :

The three-element type Ineq is used to represent the strength of a comparison between terms.

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max R1 R2 computes the strength of the sum of two inequalities. If t1 R1 0 and t2 R2 0, then t1 + t2 (max R1 R2) 0.

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Ineq is ordered eq < le < lt.

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Prints an Ineq as the corresponding infix symbol.

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Finds the name of a multiplicative lemma corresponding to an inequality strength.

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### Comparisons with 0 #

structure Linarith.Comp :
• The strength of the comparison, <, ≤≤, or =.

• The coefficients of the comparison, stored as list of pairs (i, a), where i is the index of a recorded atom, and a is the coefficient.

The main datatype for FM elimination. Variables are represented by natural numbers, each of which has an integer coefficient. Index 0 is reserved for constants, i.e. coeffs.find 0 is the coefficient of 1. The represented term is coeffs.sum (λ ⟨k, v⟩, v * Var[k]). str determines the strength of the comparison -- is it < 0, ≤ 0, or = 0?

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c.vars returns the list of variables that appear in the linear expression contained in c.

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c.coeffOf a projects the coefficient of variable a out of c.

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c.scale n scales the coefficients of c by n.

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Comp.add c1 c2 adds the expressions represented by c1 and c2. The coefficient of variable a in c1.add c2 is the sum of the coefficients of a in c1 and c2.

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Comp has a lex order. First the ineqs are compared, then the coeffs.

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A Comp represents a contradiction if its expression has no coefficients and its strength is <, that is, it represents the fact 0 < 0.

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### Control #

• The name of the preprocessor, used in trace output.

name : String
• Replace a hypothesis by a list of hypotheses. These expressions are the proof terms.

transform :

A preprocessor transforms a proof of a proposition into a proof of a different propositon. The return type is List Expr, since some preprocessing steps may create multiple new hypotheses, and some may remove a hypothesis from the list. A "no-op" preprocessor should return its input as a singleton list.

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• The name of the global preprocessor, used in trace output.

name : String
• Replace the collection of all hypotheses with new hypotheses. These expressions are proof terms.

transform :

Some preprocessors need to examine the full list of hypotheses instead of working item by item. As with Preprocessor, the input to a GlobalPreprocessor is replaced by, not added to, its output.

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Some preprocessors perform branching case splits. A Branch is used to track one of these case splits. The first component, an MVarId, is the goal corresponding to this branch of the split, given as a metavariable. The List Expr component is the list of hypotheses for linarith in this branch.

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• The name of the global branching preprocessor, used in trace output.

name : String
• Given a goal, and a list of hypotheses, produce a list of pairs (consisting of a goal and list of hypotheses).

transform :

Some preprocessors perform branching case splits. A GlobalBranchingPreprocessor produces a list of branches to run. Each branch is independent, so hypotheses that appear in multiple branches should be duplicated. The preprocessor is responsible for making sure that each branch contains the correct goal metavariable.

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A Preprocessor lifts to a GlobalPreprocessor by folding it over the input list.

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A GlobalPreprocessor lifts to a GlobalBranchingPreprocessor by producing only one branch.

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• = { name := pp.name, transform := fun g l => do let __do_lift ← pure [(g, __do_lift)] }

process pp l runs pp.transform on l and returns the result, tracing the result if trace.linarith is on.

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A CertificateOracle is a function produceCertificate : List Comp → Nat → MetaM (HashMap Nat Nat)→ Nat → MetaM (HashMap Nat Nat)→ MetaM (HashMap Nat Nat). produceCertificate hyps max_var tries to derive a contradiction from the comparisons in hyps by eliminating all variables ≤ max_var. If successful, it returns a map coeff : Nat → Nat→ Nat as a certificate. This map represents that we can find a contradiction by taking the sum ∑ (coeff i) * hyps[i]∑ (coeff i) * hyps[i].

The default CertificateOracle used by linarith is Linarith.FourierMotzkin.produceCertificate.

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• Discharger to prove that a candidate linear combination of hypothesis is zero.

discharger :
• Prove goals which are not linear comparisons by first calling exfalso.

exfalso : Bool
• Transparency mode for identifying atomic expressions in comparisons.

• Split conjunctions in hypotheses.

split_hypotheses : Bool
• Split ≠≠ in hypotheses, by branching in cases < and >.

split_ne : Bool
• Override the list of preprocessors.

preprocessors :
• Specify an oracle for identifying candidate contradictions. The only implementation here is Fourier-Motzkin elimination.

oracle :

A configuration object for linarith.

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cfg.updateReducibility reduce_default will change the transparency setting of cfg to default if reduce_default is true. In this case, it also sets the discharger to ring!, since this is typically needed when using stronger unification.

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### Auxiliary functions #

These functions are used by multiple modules, so we put them here for accessibility.

getRelSides e returns the left and right hand sides of e if e is a comparison, and fails otherwise. This function is more naturally in the Option monad, but it is convenient to put in MetaM for compositionality.

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parseCompAndExpr e checks if e is of the form t < 0, t ≤ 0≤ 0, or t = 0. If it is, it returns the comparison along with t.

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helper function for error message

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mkSingleCompZeroOf c h assumes that h is a proof of t R 0. It produces a pair (R', h'), where h' is a proof of c*t R' 0. Typically R and R' will be the same, except when c = 0, in which case R' is =. If c = 1, h' is the same as h -- specifically, it does not change the type to 1*t R 0.

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