function c = plus(a,b)% overloading of operator +% $Id: plus.m 39 2004-08-06 09:05:50Z mairas $if isa(a,'spectrum') & isnumeric(b)  c = spectrum(a.s + b, a.frequency);elseif isnumeric(a) & isa(b,'spectrum')  c = spectrum(a + b.s, b.frequency);elseif isa(a,'spectrum') & isa(b,'spectrum')  if not(a.frequency==b.frequency)    error('Frequency vectors do not match.');  end  c = spectrum(a.s+b.s,a.frequency);end