How can I get the values of XML nodes depending on the content of another node by using XSLT?

That's my very first question according to XSLT. I would like to transform a XML structure by using XSLT.

The relevant part of my XML source:

<field tag="ElementName">
  <subfield code="S">p</subfield>
  <subfield code="a">Content 1 for S=P</subfield>
</field>
<field tag="ElementName">
  <subfield code="S">p</subfield>
  <subfield code="a">Content 2 for S=P</subfield>
</field>
<field tag="ElementName">
  <subfield code="S">p</subfield>
  <subfield code="a">Content 3 for S=P</subfield>
</field>
<field tag="ElementName">
  <subfield code="S">g</subfield>
  <subfield code="a">Content 1 for S=G</subfield>
</field>
<field tag="ElementName">
  <subfield code="S">g</subfield>
  <subfield code="a">Content 2 for S=G</subfield>
</field>
<field tag="ElementName">
  <subfield code="S">s</subfield>
  <subfield code="a">Content 1 for S=S</subfield>
</field>
<field tag="ElementName">
  <subfield code="S">s</subfield>
  <subfield code="a">Content 2 for S=S</subfield>
</field>
<field tag="ElementName">
  <subfield code="S">s</subfield>
  <subfield code="a">Content 3 for S=S</subfield>
</field>
<field tag="ElementName">
  <subfield code="S">s</subfield>
  <subfield code="a">Content 4 for S=S</subfield>
</field>

Next what I would like the XML to look like after transformation:

<field tag="ElementName_P">
  <subfield code="a">Content 1 for S=P</subfield>
  <subfield code="a">Content 2 for S=P</subfield>
  <subfield code="a">Content 3 for S=P</subfield>
</field>
<field tag="ElementName_G">
  <subfield code="a">Content 1 for S=G</subfield>
  <subfield code="a">Content 2 for S=G</subfield>
</field>
<field tag="ElementName_S">
  <subfield code="a">Content 1 for S=S</subfield>
  <subfield code="a">Content 2 for S=S</subfield>
  <subfield code="a">Content 3 for S=S</subfield>
  <subfield code="a">Content 4 for S=S</subfield>
</field>

I know how to get the attribute values and the field values using xsl:variable and xsl:value-of. What I would like to know is how to group the contents from the subfields with code="a". Have been trying for hours. Are there any possibilities using XPath?

Thx for support!