Update header.tex

Author: khanh101

assets/header.tex

@@ -61,6 +61,7 @@
 \DeclareMathOperator{\Top}{Top}
 \DeclareMathOperator{\Meas}{Meas}
 \DeclareMathOperator{\Grp}{Grp}
+\DeclareMathOperator{\Op}{Op}
 \DeclareMathOperator{\Ab}{Ab}
 \DeclareMathOperator{\Ch}{Ch}
 \DeclareMathOperator{\Fun}{Fun}