I thought that I could write regular Pipeline within a declarative pipeline stage, and asked about it here:
http://stackoverflow.com/questions/39832862/jenkins-cannot-define-variable-in-pipeline-stage
After posting it in IRC I was directed to https://github.com/jenkinsci/pipeline-model-definition-plugin/blob/master/SYNTAX.md#declarative-subset-of-groovy which explained the limitations.
@abayer suggested I created this Jira ticket for clearer documentation.
Bulk closing resolved issues.