Antiparallel lines

In geometry, two lines $l_{1}$ and $l_{2}$ are antiparallel with respect to a given line $m$ if they each make congruent angles with $m$ in opposite senses. More generally, lines $l_{1}$ and $l_{2}$ are antiparallel with respect to another pair of lines $m_{1}$ and $m_{2}$ if they are antiparallel with respect to the angle bisector of $m_{1}$ and $m_{2}.$

In any cyclic quadrilateral, any two opposite sides are antiparallel with respect to the other two sides.

Lines $l_{1}$ and $l_{2}$ are antiparallel with respect to the line $m$ if they make the same angle with $m$ in the opposite senses.	Two lines $l_{1}$ and $l_{2}$ are antiparallel with respect to the sides of an angle if they make the same angle $\angle APC$ in the opposite senses with the bisector of that angle.
Given two lines $m_{1}$ and $m_{2}$ , lines $l_{1}$ and $l_{2}$ are antiparallel with respect to $m_{1}$ and $m_{2}$ if $\angle 1=\angle 2$ .	In any quadrilateral inscribed in a circle, any two opposite sides are antiparallel with respect to the other two sides.