entities.dtd

<!ENTITY fill.label "Fill">

fluent.ftl

fill =
  .label = 
    { PLATFORM() ->
      [linux] Zoom
     *[windows] Fill
    }

migration.py

# coding=utf8

from __future__ import absolute_import
import fluent.syntax.ast as FTL
from fluent.migrate.helpers import transforms_from
from fluent.migrate import COPY

def migrate(ctx):
    """Bug 123456 - Blah, part {index}."""

    ctx.add_transforms(
        "fluent.ftl",
        "fluent.ftl",
        transforms_from(
"""
fill =
  .label = { COPY(from_path, "fill.label") }
""", from_path="entities.dtd")
)

    ctx.add_transforms(
        "fluent.ftl",
        "fluent.ftl",
        [
            FTL.Message(
                id=FTL.Identifier('fill'),
                attributes=[
                    FTL.Attribute(
                        FTL.Identifier('label'),
                        FTL.Pattern(
                            elements=[
                                FTL.Placeable(
                                    expression=FTL.SelectExpression(
                                        expression=FTL.CallExpression(
                                            callee=FTL.Function('PLATFORM')
                                        ),
                                        variants=[
                                            FTL.Variant(
                                                key=FTL.VariantName('linux'),
                                                default=False,
                                                value=''
                                            ),
                                            FTL.Variant(
                                                key=FTL.VariantName('windows'),
                                                default=True,
                                                value=COPY(
                                                    'entities.dtd',
                                                    'fill.label'
                                                )
                                            )
                                        ]
                                    )
                                )
                            ]
                        )
                    )
                ]
            )
        ]