DiscreteTimeModelQ[expr] gives True if expr is a discrete-time StateSpaceModel or TransferFunctionModel object and False otherwise.
DotEqual[x, y, ...] displays as x \[DotEqual] y \[DotEqual] ....
DoubleBracketingBar[x, y, ...] displays as \[LeftDoubleBracketingBar]x, y, ...\[RightDoubleBracketingBar].
DoubleDownArrow[x, y, ...] displays as x\[DoubleDownArrow]y....
DoubleLeftArrow[x, y, ...] displays as x \[DoubleLeftArrow] y \[DoubleLeftArrow] ....
DoubleLeftRightArrow[x, y, ...] displays as x \[DoubleLeftRightArrow] y \[DoubleLeftRightArrow] ....
DoubleLongLeftArrow[x, y, ...] displays as x\[DoubleLongLeftArrow]y\[DoubleLongLeftArrow]....
DoubleLongLeftRightArrow[x, y, ...] displays as x\[DoubleLongLeftRightArrow]y\[DoubleLongLeftRightArrow]....
DoubleLongRightArrow[x, y, ...] displays as x\[DoubleLongRightArrow]y\[DoubleLongRightArrow]....
DoubleRightArrow[x, y, ...] displays as x \[DoubleRightArrow] y \[DoubleRightArrow] ....