Def 87: [Derivative of function of a real variable]
	Der(f) := arb({df in RF | domain(f) = domain(df) & 
						is_continuous_REnF(DD(f,df) ON  (domain(f) PROD domain(f)),2)})

Def 95: [Derivative of function of a complex variable]
	CDer(f) := arb({df in CF | domain(f) = domain(df) & 
						is_continuous_CEnF(CDD(f,df) ON  (domain(f) PROD domain(f)),2)})
