Use of level tag + negative 0.5 increments

[bxl-forever]

What steps will reproduce the problem?

1. Create a way with highway=steps + incline=up + level=-1;-0.5

What is the expected result?

That should be accepted without a warning, in the same way that level=0;0.5 is accepted

What happens instead?

JOSM triggers a warning: "level should have numbers only with optional .5 increments"

Please provide any additional information below. Attach a screenshot if possible.

Why do we want to use negative .5 increments? Indoor mapping of subway stations: the stairs connect level=-1 and the ground floor but those staircases are split in two parts with an intermediate landing. Using level=-0.5 for the landing and level=-1;-0.5 and level=-0.5;0 for the stairs is the most logical approach.
According to the wiki, .5 increments are allowed. It seems that JOSM has some trouble with negative values.

URL:http://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2017-02-02 01:22:52 +0100 (Thu, 02 Feb 2017)
Build-Date:2017-02-02 02:33:28
Revision:11526
Relative:URL: ^/trunk

Identification: JOSM/1.5 (11526 en) Linux Arch Linux
Memory Usage: 937 MB / 1751 MB (460 MB allocated, but free)
Java version: 1.8.0_121-b13, Oracle Corporation, OpenJDK 64-Bit Server VM
Screen: :0.0 1366x768
Maximum Screen Size: 1366x768
VM arguments: [-Djosm.restart=true]
Dataset consistency test: No problems found

Plugins:
+ measurement (33088)
+ pt_assistant (33108)
+ public_transport (33088)
+ public_transport_layer (33088)

Tagging presets:
+ https://josm.openstreetmap.de/josmfile?page=Presets/BENELUX&zip=1

Map paint styles:
- https://josm.openstreetmap.de/josmfile?page=Styles/PublicTransport&zip=1

Last errors/warnings:
- W: Unable to get color from '' for color preference 'extrude.main.line'
- W: Unable to get color from '' for color preference 'improve.way.accuracy.helper.line'
- W: Unable to get color from '' for color preference 'make.parallel.helper.line'
- W: java.net.SocketTimeoutException: connect timed out
- E: java.net.SocketTimeoutException: connect timed out
- E: org.openstreetmap.josm.io.OsmTransferException: Could not connect to the OSM server. Please check your internet connection.. Cause: java.net.SocketTimeoutException: connect timed out
- W: java.net.SocketTimeoutException: connect timed out
- W: Already here java.net.SocketTimeoutException: connect timed out
- E: java.net.SocketTimeoutException: connect timed out
- W: org.openstreetmap.josm.io.OsmTransferException: Could not connect to the OSM server. Please check your internet connection.. Cause: java.net.SocketTimeoutException: connect timed out

[Klumbumbus]

In 11640/josm:

fix #14427 - allow negative .5 increments in level tag, asserts, docu