Workflow to make a field "Read Only" after modification
                    I'm trying to figure out how to create a workflow for Opportunities that will make a field Read Only after it's been modified.  This could be for a new Opportunity or existing Opportunity.  In the case of a new Opportunity, if this said field has a value placed into it, then this field becomes a Read Only field.  Any ideas?  Can this be done?
                -rs
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