TAPE GAP DENIAL BY NIXON IS CITED
Nixon atty Herbert J Miller Jr says, during oral arguments in Fed Ct, Washington, DC, over constitutionality of new statute that gives Govt control over Nixon's Pres papers and tapes, that ex-Pres Nixon denied under oath 'responsibility' for 18 1/2-min gap in key White House tape; says Nixon made denial when he gave 11 hrs of grand jury testimony in Calif last June under questioning by Watergate prosecutor's office; Nixon is challenging statute as unconst; Justice Dept is defending statute (M)