Denial of a substantial right at the preliminary hearing renders a defendant's commitment illegal and entitles him to a dismissal of the information on timely motion. (People v. Pompa-Ortiz (1980) 27 Cal. 3d 519, 523 165 Cal. Rptr. 851, 612 P.2d 941; Stanton v. Superior Court (1987) 193 Cal. App. 3d 265, 270 239 Cal. Rptr. 328). Deprivation of a substantial right is properly addressed by a section 995 motion when the error is visible from the "four corners" of the preliminary hearing transcript. By contrast, an error that is not known or visible at the hearing itself (such as the competency issue in this case), may be called to the court's attention through a nonstatutory motion to dismiss. ( Merrill v. Superior Court (1994) 27 Cal. App. 4th 1586, 1595-1596 33 Cal. Rptr. 2d 515; Stanton, supra, at p. 269). The trial court considers the materiality of the nondisclosed information and determines its effect on probable cause and the legality of the commitment proceedings. (Merrill, supra, at p. 1596). There appears to be an exception to the general rule that a section 995 motion may only be brought when the issue of competency is contained within the four corners of the transcript. (See Bayramoglu v. Superior Court (1981) 124 Cal. App. 3d 718, 728-729 176 Cal. Rptr. 487 995 motion granted where defendant presented additional evidence showing that he was incompetent at the time of the preliminary hearing.) Given the posture of this case, we need not resolve whether a section 995 motion was the appropriate vehicle for setting aside the information. ( Merrill v. Superior Court, supra, 27 Cal. App. 4th at p. 1596.) "Use of the nonstatutory or pretrial motion to dismiss has been sanctioned by our Supreme Court. 'Although no clear California statutory authority provides for such a pretrial motion to dismiss, we have no doubt in light of the constitutional nature of the issue as to the trial court's authority to entertain such a claim.' ( Murgia v. Municipal Court (1975) 15 Cal. 3d 286, 294, fn. 4 124 Cal. Rptr. 204, 540 P.2d 44). A pretrial nonstatutory motion to dismiss is now accepted as an appropriate vehicle to raise a variety of defects." ( Stanton v. Superior Court, supra, 193 Cal. App. 3d at p. 271).