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# Partial Metafunction Application

Consider the lambda expression `mpl::plus<_1,_1>`. A single argument is directed to both of `plus`'s parameters, thereby adding a number to itself. Thus, a binary metafunction, `plus`, is used to build a unary lambda expression. In other words, we've created a whole new computation! We're not done yet, though: By supplying a non-placeholder as one of the arguments, we can build a unary lambda expression that adds a fixed value, say 42, to its argument:

    mpl::plus<_1, mpl::int_<42> >

The process of binding argument values to a subset of a function's parameters is known in the world of functional programming as partial function application.