Instantly, such presumption has not been rebutted since there has been no proof that appellant's withdrawal of his appeal was anything but a `knowing and understanding' one. Commonwealth v. Via, [ 445 Pa. 373 EQD, A.2d 895 (1974)]. Furthermore, where, as here, appellant has been represented by private counsel every step of the appeal process, in the absence of proof of extraordinary circumstances, see Section 4(b)(2), such a waiver is considered binding.