Imposing requiredness on the xsd element

Team,

Can we impose required field concept in xsd? I am looking for the exact 
equivalent of
<!ELEMENT note (to,from?,heading?,body?)>
<!ELEMENT to      (#PCDATA)>
<!ELEMENT from    (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body    (#PCDATA)>

Here "to" is the required element where as "From", "heading" and "Body" are 
optional fields. I don't want to make this requirement imposed on attribute 
in xsd, but I want this as element. How can I achieve this?

Thank you.

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