directed and undirected edge in the same graph

Is it possible to have both kinds of edges?
Directed Edges:
A1 -> B1 -> C1 -> D1
A2 -> B2 -> C2 -> D2

Undirected/symmetric edges rendered with arrow-heads on both-ends:

A1 <-> A2
B1 <-> B2
C1 <-> C2
D1 <-> D2

At present, this is not

At present, this is not possible grammatically. If you declare a graph as a digraph G { }, all of its edges must use ->; if it is just graph G {}, then all of the edges must use --. Note though that this is just a way of being clear concerning what type of graph you are working with and what are the appropriate defaults. You can use the dir, arrowhead and arrowtail attributes to make edges look any way you want. For example,

  C1 -> C2 [dir=both]   // You'll see C1 <-> C2

  C1 -> C2 [dir=none] // You'll see C1 -- C2

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