#variables ...........
  #..................... 
  init = tf.initialize_all_variables()
  sess.run(init)

  #variables ...
  ...
  init = tf.variables_initializer([b,c])
  sess.run(init)

  #definition of variables a, b, c ...
  ....
  my_optimizer = tf.train.RMSProp(learning_rate = 0.1).minimize(my_cost)
  init = tf.variables_initializer([b,c])
  sess.run(init)

  a = tf.Variables(...)      #line N
  temp = set(tf.all_variables()) 
  b = tf.Variables(...)
  c = tf.Variables(...) 
  #definition of my optimizer
  optimizer = tf.train.......
  init = tf.variables_initializer(set(tf.all_varialbles())-temp) # line M
  sess.run(init)