Compound operator with built-in evaluation rules.
mi,j, ... is by default interpreted as Part[m, i, j, ... ].
Sometimes used in mathematics to indicate a class of algebraic objects with certain variables or extensions.
Extensible character; grows by default to limited size.
See Section 1.10.4 and Section 3.10.4.
See also: \[RawLeftBracket] , \[LeftDoubleBracketingBar] .