Determining an error in position of at least two heads coupled to a piezoelectric material, and changing a potential applied to a piezoelectric material to reduce the error in position of the at least two heads. The shield may be provided as an elongated flexible sheath having one or more thin flexible sidewalls that can conform to the tissue around or bearing on the shield. A pipe is secured on a vertical sucking tap hole. The first part includes a base having upstanding tissue piercing posts, and the second part includes another base defining apertures for receiving the posts, as well as a portion movable relative to the second base. The invention further relates to a composition suitable for the preparation of a printing plate by inkjet printing including an aqueous solution or aqueous colloidal dispersion of a polymer having water-solubilising groups characterised in that said aqueous solution or aqueous colloidal dispersion further includes pigment particles dispersed therein.