Returns the arc-tangent value, in radians, of the given numerical argument
Implemented by: dartmouth2, ansimin, ansifull, hptsb, decbasic, altair8K, Applesoft, TI994, CBM, Atari, msatari, level2, colorext, zx81, spectrum, BBC, GWBASIC, MSX, bwbasic, bw32, ABasiC, QBasic, VBDOS, FreeBASIC, minibasicjs, EndBASIC
With variations:
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