sess = tf.InteractiveSession() ## without specifying the context 'sess'.
sess.close()
with g.control_dependencies([a,b,c]):
d = ....
e = .....
Placeholders:
tf.placeholder(dtype, shape = None, name = None)
placeholder must be feed:
sess.run(c,{a:[1,2,3]})
sess.run(tf.global_variables_initializer())