Quince defines an overload of the infix binary operator +,
which builds and returns an exprn_mapper<std::string> that represents string concatenation.
It's a wrapper for the SQL operator ||.
This overload applies when:
abstract_mapper<std::string>, and
abstract_mapper<std::string> or a std::string
or a const char
*,
The returned exprn_mapper<std::string> represents a server-side expression
that, when executed, evaluates the subexpressions represented by the operands,
and concatenates their (string) results.