Article 1.15 of the Texas Code of Criminal Procedure provides that where a defendant pleads guilty and waives his right of trial by jury, "it shall be necessary for the state to introduce evidence into the record showing the guilt of the defendant . . . ." See TEX. CODE CRIM. PROC. ANN. art 1.15 (Vernon Supp. 2000). It also provides that the "evidence may be stipulated if the defendant in such case consents in writing, in open court, to waive appearance, confrontation, and cross-examination of witnesses, and further consents . . . to an oral stipulation of the evidence and testimony . . . ." See id. The waiver and consent "must be approved by the court in writing . . . ." See id.