The snippet generator suggests that the groovy code sohuld be:

input message: '', parameters: [choice(choices: ['a', 'b', 'c', 'd'], description: 'whichletter', name: 'abcd')]

However, I found that choices: expects a string, not an array.

I think that instead of

      choices: ['a','b','c','d']

the snippet generator should instead suggest 

     choices: 'a\nb\nc\nd'

Thanks for your attention to this issue.