wip4

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diff --git a/rabbit-prover/primitive.scm b/rabbit-prover/primitive.scm
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+;;; Rabbit Prover --- Prove S-expression
+;;; Copyright © 2020 Masaya Tojo <masaya@tojo.tokyo>
+;;;
+;;; This file is part of Rabbit Prover.
+;;;
+;;; Rabbit Prover is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or
+;;; (at your option) any later version.
+;;;
+;;; Rabbit Prover is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;; General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with Rabbit Prover.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (rabbit-prover primitive)
+  #:export (natural?))
+
+;; (natural? x) -> boolean?
+(define (natural? x)
+  (and (integer? x)
+       (not (negative? x))))