Root poset should treat type A1 properly

[stumpc5]

The patch fixes the issue that the one element root poset in type A1 was not returned correctly.

sage: Phi = RootSystem(['A',1]).root_poset(); Phi
Finite poset containing 1 elements
sage: Phi.cover_relations()
[]

I also added the facade option for root posets.

Component: combinatorics

Keywords: root poset

Author: Christian Stump

Reviewer: Frédéric Chapoton

Merged: sage-5.1.beta0

Issue created by migration from https://trac.sagemath.org/ticket/12838

[fchapoton]

Reviewer: Frédéric Chapoton

[fchapoton]

comment:2

In sage/combinat/root_system/root_system.py,

it should be written

- ``restricted`` -- 

(i.e. there are missing backquotes)

Otherwise, it seems ok.

[stumpc5]

comment:3

Replying to @fchapoton:

In sage/combinat/root_system/root_system.py,

it should be written

- ``restricted`` -- 

(i.e. there are missing backquotes)

Otherwise, it seems ok.

done.