Proof : Let L be a CSL. Then L = L(G) for some CSG G = ( N, 
, P, S ). we now construct a 3-tape nondeterministic TM M to simulate the derivations of G. The techniques used to construct the TM M from G is exactly the same which were used in constructing the LBA M from a CSG G in the proof of the theorem ....(to show that if L is a CSL then L = L(M) for some LBA M).
In this case there are 3-tapes (instead of a 2-track tape LBA M) - the first tape holds the input string , the second tape holds the sentential form generated by the simulated derivation and the third tape holds the entire derivation. when a production 
is applied to the sentential form on tape 3, the string 
is written on the tape following 
# , i.e. the derived sentential forms are separated by the special symbol #.