Eulerian circuits

Eulerian circuits and lines of dominoes
How many different lines with all the 28 dominoes, following the usual rules. That is two dominoes touch by the same number of points ?
The dominoes are in a line from left to right, the same string "reversed" considered as different.
(The usage says the doubles are "across", this doesn't change anything to the problem)

You may practice first with a reduced set from 0-0 to 4-4
   Solution

Or even reduced further, from 0-0 to 2-2 !
   Solution

before dealing with the full set from 0-0 to 6-6
   Solution

What do you think about an extended set from 0-0 to 9-9 ?
   Solution

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