are there any functions for computing a square root of a modulo p (prime)?

[Ramón T. B.]

Hello,

  I would like to know if there are any functions to compute a square root (Modular arithmetic) for a number, say 'a', modulo a prime number, say 'p' (prime). I mean, to solve the congruence  x² ≡ a mod p

Thanks in advance: framontb