PR 120: Allow source port on p:invisible-xml to be empty

XProc specifications published by continuous integration on 09 Apr 2026 for PR 120.

Specifications marked with ⏵ are likely to have been changed by this commit.

• XProc 3.1: An XML Pipeline Language
• XProc 3.1: Standard Step Library
• XProc 3.1: Invisible XML
• XProc 3.1: dynamic pipeline execution
• XProc 3.1: file steps
• XProc 3.1: mail steps
• XProc 3.1: operating system steps
• XProc 3.1: paged media steps
• XProc 3.1: text steps
• XProc 3.1: validation steps