function [varargout] = likExp(link, hyp, varargin)
% likExp - Exponential likelihood function for strictly positive data y. The
% expression for the likelihood is
% likExp(f) = exp(-y/mu) / mu with
% mean=mu and variance=mu^2 where mu = g(f) is the intensity, f is a
% Gaussian process, y is the strictly positive data. Hence, we have
% llik(f) = log(likExp(f)) = -y/g(f) - log(g(f)).
%
% Internally, the function is in fact a wrapper around likGamma with hyper
% parameter al=1.
%
% We provide two inverse link functions 'exp' and 'logistic':
% g(f) = exp(f) and g(f) = log(1+exp(f))).
% The link functions are located at util/glm_invlink_*.m.
% Note that for the 'exp' intensity the likelihood lik(f) is log concave.
%
% The hyperparameters are:
%
% hyp = [ ]
%
% Several modes are provided, for computing likelihoods, derivatives and moments
% respectively, see likFunctions.m for the details. In general, care is taken
% to avoid numerical issues when the arguments are extreme.
%
% See also LIKFUNCTIONS.M, LIKGAMMA.M.
%
% Copyright (c) by Hannes Nickisch, 2013-10-29.
if nargin<4, varargout = {'0'}; return; end % report number of hyperparameters
varargout = cell(nargout, 1); % allocate the right number of output arguments
for j=1:nargout, varargout{j} = []; end % derivative output
if nargin<=6, [varargout{:}] = likGamma(link,0,varargin{:}); end % log(al) = 0