cos(0) should be 1

[john-blackburn]

f24/f24cos.z80 says cos(0)==0 in the comments. But cos(0)=1

[john-blackburn]

In my code I have changed the first few lines of this routine to

f24cos:
;cos(AHL) ==> AHL

;cos(0) == 1    ; fixed by JFB
  add a,a
  jr nz,f24cos_skip
  ld a,$3F ; 1.0
  ld hl,0
  ret

;cos(inf)==cos(NaN)==NaN
f24cos_skip:
  rrca
  cp $7F

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