PRO vcorr, w, f, v, mode

; Apply a velocity correction to a wavelength array
; v is in units of the speed of light

IF abs(mode) EQ  1 THEN w = w / (1.0 + v)
IF abs(mode) EQ  2 THEN w = w * (1.0 + v)

IF     mode  EQ -1 THEN f = f * (1.0 + v)
IF     mode  EQ -2 THEN f = f / (1.0 + v)  

END