Example of spectral feature removal and masking in a HR21 spectrum. The black line stands for the original HR21 spectrum of CNAME 23354061-4305405 (MJD = 56856.376009; median S/N ≈ 30); the red line stands for the modified version of the spectrum, with the Ca II triplet cancelled out and the telluric bands masked while the Mg I line at 8806 Å is left untouched because the criteria were not met (see text). There is a small shift between the vertical marks and the position of the absorption lines since the spectrum is not corrected for the stellar radial velocity (the spectrum is in the heliocentric frame); our algorithm performs a small exploration around the laboratory wavelength to find the observed position of the calcium and magnesium lines to be removed.