Definition: atom x2 = sqrt(atom x1)
Description: Calculate the square root of x1.
Comments: This function may be applied to an atom or sq_sqrt() to all elements of a sequence. The rules for sequence operations apply.

Taking the square root of a negative number will abort your program with a run-time error message.
r = sqrt(16)
-- r is 4
Implementation: via :%opSqrt in builtins\VM\pTrig.e (an autoinclude)
See Also: log, power