code: def withdraw_consent(consent_tier=nil)
        if consent_tier.nil? then
          return specimen_collection_group.consent_tier_statuses.each { |cts| withdraw_consent(cts.consent_tier) }
        end
        tgt = consent_tier_statuses.detect { |cts| cts.consent_tier.matches?(consent_tier) }
        if tgt.nil? then
          consent_tier_statuses << tgt = ConsentTierStatus.new(:consent_tier => consent_tier)
        end
        tgt.status = 'Withdrawn'
      end
docs: Withdraws consent for this Specimen as follows:
   * If a consent tier is provided, then the SCG {CaTissue::ConsentTierStatus} with the
     given consent tier is withdrawn.
   * Otherwise, all SCG consents are withdrawn.
  
   @param [CaTissue::ConsentTier, nil] consent_tier the optional consent tier to withdraw
   @raise [Jinx::ValidationError] if there is no SCG consent status for the given consent tier