In Guided Rule Editor with DSL option, you can add "From Accumulate" but "Function"/"Custom Code" gets an error "Unable to expand".

Reproduce steps:

• Unzip the attached reproducer-repo.zip
• Clone the repo via business-central "Administration" > "Repositories" > "Clone Repository"
• Go to Project Explorer, choose the repo, choose "project1"
• Go to package org > kie > example > project1. You will find DataObject 'Person,' DSL, Test Scenario
• Create a new Guided Rule. Check "Use Domain Specific Language (DSL)" option
• Add conditions (See guidedrule01.png)
  • DSL "There is a person whose age is 20"
  • "From Accumulate ..."
    • Choose "Number" for the first "click to add pattern..."
    • Choose "Person" for the second "click to add pattern..."
    • Configure Variable name "$p" for "All Person with"
  • Write "count($p)" in Function
• Save
• Press "Validate"
• You will get an error "Unable to expand" (See error01.png)

This is because "count($p)" line doesn't have preceding ">".

rule "gr1"
        dialect "mvel"
        when
                There is a person whose age is 20
                >Number( ) from accumulate ( $p : Person( ),
                        count($p))
        then
end

• If you add ">" at the begging of Function (See guidedrule02.png), you don't get the error.
• But if you close the Editor and reopen the rdslr, the condition is rendered as free form DRL (See guidedrule03.png). So it cannot be a workaround.

• "Custom Code" has the same problem as "Function". Furthermore, "Custom Code" cannot use ">" because ">" wouldn't be placed at the beginning of the line.