error("U must be a column vector with n_u*n_U components");
    endif
  else
    n_u = length(U)/n_U;	# Deduce n_u from U if no system
  endif
  
  [n_x0,m_x0] = size(x_0);
  if n_x0<>n_x
  if n_x0!=n_x
    error(sprintf("x_0 must be a column with length %i", n_x));
  endif
  

  [n,m]=size(tau);
  if (n != 1 )
    error("tau must be a row vector of times");