function [ x ] = backward_substitutions( U, y )
% BACKWARD_SUBSTITUTIONS solve the linear system U x = y by means of the
% backward subsitutions algorithm; U must be an upper triangular matrix
%  [ x ] = backward_substitutions( U, y )
%  Inputs: U = upper triangular matrix (square matrix)
%          y = vector (right hand side of the linear system)
%  Output: x = solution vector (column vector)
%

return