x1 is the result of evaluating mathematical object x2 at point/with input value x3 (ordered tuple)
Currently, partial evaluation is not supported. x2 should be a mathematical function which has/is defined with indeterminates (such as a polynomial (in the abstract algebraic sense: a finite and ordered tuple of coefficients from an algebraic ring)) or a function. x1 is a concrete, explicit, and specific object with a single value; although this might be called a number, it need not be a scalar; it could be a scalar, a vector, a point, a more general tensor, or many other things. It itself could even be function, if we understand x2 to be a function-valued map with its domain being an index telling which function it is (such as is the case, sort of, with homologies).