Families of lines and concurrency

Build tools that meet two lines exactly, find the member of a family through a point, and test three lines for concurrency.

The idea. A line $Ax + By + C = 0$ is stored as the triple (A, B, C). Two lines meet at one point unless they are parallel. The line $L_1 + kL_2 = 0$ passes through a given point when $k = -\dfrac{L_1(\text{point})}{L_2(\text{point})}$. Three lines are concurrent when the meeting point of two lies on the third.

Your tasks.

1. In meet, return the meeting point of two lines as a pair of fractions, or None if they are parallel.
2. In family_k, return the number $k$ for which $L_1 + kL_2 = 0$ passes through the point, or None when $L_2$ is $0$ at the point.
3. In concurrent, return True when the three lines pass through one point, and False otherwise.