Cannot convert list to _dynet.Expression

I want to run an example with dynet.concatenate_cols()
I would like to use a list, to check the behavior. For example:

task_predictions = [[1,2], [3,4]]
dynet.concatenate_cols(task_predictions)

But I take an error Cannot convert list to _dynet.Expression

How am I supposed to initialize the object that concatenate_cols wants?
Thanks

edit: I used the following as an example, which is not equavalant with the above
(but I also cannot see the output)
Question: Is there any running mode of dynet that I could see the output?

example:
-----------

dimension = 3
v1 = dynet.vecInput(dimension)
v1.set([1,2,3])

v2 = dynet.vecInput(dimension)
v2.set([11,12,13])
v1_l = list(v1)
v2_l = list(v2)
v1v2 = v1_l+v2_l
res = dynet.concatenate_cols(v1v2)
print (res)

--
**expression 42/0**

Start with the issue's concatenate_cols() examples and inspect the DyNet Python API documentation or binding entry point for the accepted inputs. Clarify how a list should be converted or initialized, and document how users can inspect the resulting expression or output.

Written by the indexing model from the issue text.