README.md: Add basic documentation.

Author: roberthoenig

README.md

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-# FirstOrderLogic
+# FirstOrderLogic.jl
 
 This is a Julia package for parsing, manipulating and evaluating formulas in first-order logic.
 
+## Overview
+
+`FirstOrderLogic.jl` follows the standard structure for Julia projects.
+
+### Functionality
+
+`FirstOrderLogic.jl` currently provides the following:
+
+* An intuitive type system for formulas in first-order logic.
+
+* Various methods for converting formulas in special forms:
+    * prenex normal form
+    * skolem normal form
+    * conjunctive normal form
+    * ...
+
+* A prover for formulas in first-order logic.
+
+### Purpose
+
+`FirstOrderLogic.jl` may prove useful for the following:
+
+* Easily verifying the unsatisfiability of short formulas in first-order logic.
+    * The `is_satisfiable()` function is made for this very purpose.
+
+* Learning basic first-order logic concepts and algorithms.
+    * The whole code base is well-designed and documented.
+
+* *Coming soon!* Quickly verifying the unsatisfiability of long formulas in first-order logic.
+    * A highly optimized, parallelizable version of `is_satisfiable()` is in progress.
+
+## Installation
+
+1. Run
+```
+julia
+```
+2. In your Julia shell, run
+```
+Pkg.clone("git@github.com:roberthoenig/FirstOrderLogic.jl.git")
+```
+
+## Usage
+
+### Quick walkthrough
+
+Say we have a formula `∀x(P(x) ∧ ¬P(f(x)))`. We want to verify that this formula is unsatisfiable.
+To do so, we do the following:
+1. Type the formula in `FirstOrderLogic.jl` syntax: `*{x}(P(x) & P(f(x)))`.
+2. Run
+```
+julia
+```
+3. Import `FirstOrderLogic.jl`:
+```
+using FirstOrderLogic
+```
+4. Run `is_satisfiable(<our formula>)`:
+```
+is_satisfiable("*{x}(P(x) & ~P(f(x)))")
+```
+If everything works correctly, the program should print "false" and terminate. This means that `∀x(P(x) ∧ ¬P(f(x)))`
+is unsatisfiable. We have verified what we wanted to verify.
+
+### Formula syntax
+
+`FirstOrderLogic.jl` currently uses its own syntax for representing formulas
+in first-order logic. A parser for latex symbols is in progress.
+
+The following is a list of all currently supported syntactic elements.
+
+| Mathematical syntax | `FirstOrderLogic.jl` syntax | Semantic |
+| ------------------- |:------------------------:| --------:|
+  x                   | x                        | variable
+  f(x,y)              | f(x,y)                   | function
+  f(x,y)              | f(x,y)                   | predicate
+  ∃x                  | ?{x}                     | existential quantification
+  ∀x                  | *{x}                     | universal quantification
+  ¬x                  | ~x                       | negation
+  x ∧ y               | x & y                    | conjunction
+  x ∨ y               | x &#124; y               | disjunction
+  (x ∧ y) ∨ z         | (x & y) | z              | precedence grouping
+
+#### Example
+
+Formula in mathematical notation:
+```
+∀x∃yA(x,g(y,z)) ∧ (¬B() ∨ ∃dC(d))
+```
+The same formula in `FirstOrderLogic.jl` notation:
+```
+*{x}?{y}A(x,g(y,z)) & (~B() | ?{d}C(d))
+```
+
+### Some important functions
+
+| Function | Purpose |
+|:--------:|:-------:|
+ `is_satisfiable(formula; maxsearchdepth)` | Checks if `formula` is satisfiable.
+ `get_conjunctive_normal_form(formula)` | Returns `formula` in the conjunctive normal form, expressed as clauses.
+ `get_prenex_normal_form(formula)` | Returns the prenex normal form of `formula`.
+ `get_skolem_normal_form(formula)` | Returns the skolem normal form of `formula`.
+ `Formula(str)` | Parse str to a `Formula` object and return that object.
+
+Each available function comes with a concise docstring.
+
+## How is this different from PROLOG?
+
+The PROLOG programming language can be used to prove seemingly arbitrary statements in first-order logic. PROLOG differs from `FirstOrderLogic.jl` in two ways:
+* Syntax: PROLOG code is written in in facts. These facts represent horn clauses. In doing so,
+  PROLOG hides the first-order logic syntax from the user. This proves convenient for real
+  world applications, but it's not mathematically pure. `FirstOrderLogic.jl` uses a direct
+  mathematical syntax for expressing formulas.
+* Expressive power: PROLOG works on a limited set of all valid formulas in first-order logic, namely
+  only formulas that can be expressed as a conjunction of [horn clauses]
+  (https://en.wikipedia.org/wiki/Horn_clause). This is sufficient for many real world applications,
+  but it's not mathematically complete. PROLOG thereby trades completeness for computation time.
+  `FirstOrderLogic.jl` does not make that trade. It is slow, but complete.
+
 [codecov-img]: https://codecov.io/gh/roberthoenig/FirstOrderLogic.jl/branch/master/graph/badge.svg
 [codecov-url]: https://codecov.io/gh/roberthoenig/FirstOrderLogic.jl
 [travis-img]: https://travis-ci.org/roberthoenig/FirstOrderLogic.jl.svg?branch=master