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diff --git a/fuzzylite/src/term/SigmoidDifference.cpp b/fuzzylite/src/term/SigmoidDifference.cpp
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+/*
+ Author: Juan Rada-Vilela, Ph.D.
+ Copyright (C) 2010-2014 FuzzyLite Limited
+ All rights reserved
+
+ This file is part of fuzzylite.
+
+ fuzzylite is free software: you can redistribute it and/or modify it under
+ the terms of the GNU Lesser General Public License as published by the Free
+ Software Foundation, either version 3 of the License, or (at your option)
+ any later version.
+
+ fuzzylite 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 Lesser General Public License
+ for more details.
+
+ You should have received a copy of the GNU Lesser General Public License
+ along with fuzzylite.  If not, see <http://www.gnu.org/licenses/>.
+
+ fuzzylite™ is a trademark of FuzzyLite Limited.
+
+ */
+
+#include "fl/term/SigmoidDifference.h"
+
+namespace fl {
+
+    SigmoidDifference::SigmoidDifference(const std::string& name,
+            scalar left, scalar rising,
+            scalar falling, scalar right, scalar height)
+    : Term(name, height), _left(left), _rising(rising), _falling(falling), _right(right) {
+    }
+
+    SigmoidDifference::~SigmoidDifference() {
+    }
+
+    std::string SigmoidDifference::className() const {
+        return "SigmoidDifference";
+    }
+
+    scalar SigmoidDifference::membership(scalar x) const {
+        if (fl::Op::isNaN(x)) return fl::nan;
+
+        scalar a = 1.0 / (1 + std::exp(-_rising * (x - _left)));
+        scalar b = 1.0 / (1 + std::exp(-_falling * (x - _right)));
+        return _height * std::abs(a - b);
+    }
+
+    std::string SigmoidDifference::parameters() const {
+        return Op::join(4, " ", _left, _rising, _falling, _right) +
+                (not Op::isEq(_height, 1.0) ? " " + Op::str(_height) : "");
+    }
+
+    void SigmoidDifference::configure(const std::string& parameters) {
+        if (parameters.empty()) return;
+        std::vector<std::string> values = Op::split(parameters, " ");
+        std::size_t required = 4;
+        if (values.size() < required) {
+            std::ostringstream ex;
+            ex << "[configuration error] term <" << className() << ">"
+                    << " requires <" << required << "> parameters";
+            throw fl::Exception(ex.str(), FL_AT);
+        }
+        setLeft(Op::toScalar(values.at(0)));
+        setRising(Op::toScalar(values.at(1)));
+        setFalling(Op::toScalar(values.at(2)));
+        setRight(Op::toScalar(values.at(3)));
+        if (values.size() > required)
+            setHeight(Op::toScalar(values.at(required)));
+    }
+
+    void SigmoidDifference::setLeft(scalar leftInflection) {
+        this->_left = leftInflection;
+    }
+
+    scalar SigmoidDifference::getLeft() const {
+        return this->_left;
+    }
+
+    void SigmoidDifference::setRising(scalar risingSlope) {
+        this->_rising = risingSlope;
+    }
+
+    scalar SigmoidDifference::getRising() const {
+        return this->_rising;
+    }
+
+    void SigmoidDifference::setFalling(scalar fallingSlope) {
+        this->_falling = fallingSlope;
+    }
+
+    scalar SigmoidDifference::getFalling() const {
+        return this->_falling;
+    }
+
+    void SigmoidDifference::setRight(scalar rightInflection) {
+        this->_right = rightInflection;
+    }
+
+    scalar SigmoidDifference::getRight() const {
+        return this->_right;
+    }
+
+    SigmoidDifference* SigmoidDifference::clone() const {
+        return new SigmoidDifference(*this);
+    }
+
+    Term* SigmoidDifference::constructor() {
+        return new SigmoidDifference;
+    }
+
+
+}