newcommand{expcti}[2]{operatorname{mathbb{E}}_{#2}expectiarg#1}
DeclarePairedDelimiterX{expectiarg}[1]{[}{]}{%
  ifnumcurrentgrouptype=16 elsebegingroupfi
  activatebar#1
  ifnumcurrentgrouptype=16 elseendgroupfi
}


newcommand{innermid}{nonscript;delimsizevertnonscript;}
newcommand{activatebar}{%
  begingrouplccode`~=`|
  lowercase{endgrouplet~}innermid
  mathcode`|=string"8000
}