Some maths

diff --git a/gacela/math.scm b/gacela/math.scm
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+;;; Gacela, a GNU Guile extension for fast games development
+;;; Copyright (C) 2016 by Javier Sancho Fernandez <jsf at jsancho dot org>
+;;;
+;;; This program 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.
+;;;
+;;; This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+(define-module (gacela math)
+  #:export (pi
+           2pi
+           pi/2
+           clamp))
+
+;;; Based on Sly code. Thank you so much!!
+
+(define pi 3.141592654)
+(define 2pi (* pi 2))
+(define pi/2 (/ pi 2))
+
+(define (clamp min max x)
+  "Restrict X to the range defined by MIN and MAX. Assumes that MIN is
+actually less than MAX."
+  (cond ((< x min) min)
+        ((> x max) max)
+        (else x)))