function varargout = implies(varargin)
%IMPLIES Logical implication
%
% IMPLIES(X,Y) creates a mixed integer representation of
% the constraint X --> Y, i.e. Y is true if X is true.
%
% Syntax
%   F = implies(X,Y,tol)
%
% Input
%   X : binary SDPVAR variable or a constraint
%   Y : binary SDPVAR variable or a constraint
%  tol: Optional threshhold for defining zero (see NOTE)
%
% Output
%   F : SET object
%
% Examples
%
%  binvar X,Y; F = set(implies(X,Y));
%  binvar X;sdpvar Y; F = set(implies(X,Y>5));
%  binvar X;Y=sdpvar(3,1); F = set(implies(X,[sum(Y);Y(2)]>[5;0]));
%
% Note
%  Using implies with X non-binary is highly sensitive numerically.
%  The problem comes from the definition of 0 in a floating-point
%  environment, and precision in the solver. To account for this,
%  the user can supply a third argument to define a dead-zone around
%  zero, i.e Implies(X>0,Y) will be replaced with IMPLIES(abs(X)<tol,Y)
%  Note, you always need to tqweak this number for your application. By
%  default, YALMIP uses tol = 0, which means you can get garbage...
%
%  The function IMPLIES is not complete, but will be
%  improved upon in future releases.
%
%   See also @SDPVAR/AND, @SDPVAR/OR, IFF

% Author Johan Löfberg
% $Id: implies.m,v 1.1 2006/05/14 13:02:34 joloef Exp $


% There are some cases to take care of...
%
% X --> Y     binary/binary :                     Implemented
% X --> Y     binary/(LP,equality,sdp)            Implemented
% X --> Y     (LP,equality,sdp)/binary            Not implemented
% X --> Y     (LP,equality,sdp)/(LP,equality,sdp) Not implemented

X = varargin{1};
Y = varargin{2};

% Call recursicely on X -> (A,B,...)
if isa(varargin{1},'sdpvar') & (isa(varargin{2},'lmi') | isa(varargin{2},'constraint'))
    if length(varargin{1})==1 & length(varargin{2}) >1
        F = set([]);
        for i = 1:length(varargin{2})
            F = F + implies(varargin{1},varargin{2}(i));
        end
        varargout{1} = F;
        return
    end
end

if nargin == 3
    tol = varargin{3};
else
    tol = 0;
end

switch class(varargin{1})

    case 'sdpvar'
        varargout{1} = set(yalmip('addextendedvariable',mfilename,varargin{:}) == 1);        
        
    case 'char'
        varargout{1} = implies_internal(varargin{3},varargin{4});
        varargout{2} = struct('convexity','milp','monotonicity','milp','definiteness','milp','extra','marker');
        varargout{3} = varargin{3};
end

