Schema Component Constraint: Element Declarations Consistent

Hi all,

I have a question on the implication of the substitution group and of the
{disallowed substitution} on the constraint  "Schema Component Constraint:
Element Declarations Consistent" [1]

Is this constraint valid for the model group definition MG bellow?

<element name="a" **block="substitution"**/>
<element name="b" substitutionGroup="a"/>

<group name="MG">
  <element ref="a"/>
  <element name="b" type="boolean"/>
</group>

And what about transitive substitution group?  Is the group MG2 bellow
valid?

<element name="x" **block="restricction"**/>
<element name="y" substitutionGroup="x"/>
<element name="z" substitutionGroup="y" type="string"/>

<group name="MG2">
  <element ref="x"/>
  <element name="z" type="boolean"/>
</group>

[1] XML Schema Part 1 - 3.8.6 - Schema Component Constraint: Element
Declarations Consistent
(http://www.w3.org/TR/xmlschema-1/#cos-element-consistent)

Thanks,

Michael Marchegay