  A variety is a class of algebraic structures axiomatized by a set of
equations. An equation is linear if there is at most one occurrence of an
operation symbol on each side. We show that a variety axiomatized by linear
equations has the strong amalgamation property.
  Suppose further that the language has no constant symbol and, for each
equation, either one side is operation-free, or exactly the same variables
appear on both sides. Then also the joint embedding property holds.
  Examples include most varieties defining classical Maltsev conditions. In a
few special cases, the above properties are preserved when further unary
operations appear in the equations.
