  In this note, we introduce the new tool reSolve, a Monte Carlo differential
cross-section and parton-level event generator whose main purpose is to add
transverse momentum resummation to a general class of inclusive processes at
hadron colliders, namely all those which do not involve hadrons or jets in the
measured final state. This documentation refers to the first main version
release, which will form the basis for continued developments, consequently it
only implements the key features of those we plan to ultimately include. This
article acts as a manual for the program; describing in detail its use,
structure, validation and results; whilst also highlighting key aspects of the
resummation formalism applied. It details the two classes of processes so far
included; these are diphoton production and Drell-Yan production.
  A main concept behind the development of the tool is that it is a hands-on
white box for the user: significant effort has been made to give the program a
modular structure, making the various parts which comprise it independent of
each other as much as possible and ensuring they are transparently documented,
customizable and, in principle, replaceable with something that may better
serve the users needs.
  reSolve is a new C++ program, based on an evolution of the private Fortran
code 2gres, previously used for the calculations in refs. [1] and [2]; it is
also influenced by the DYRes Fortran code of refs. [3] and [4]. This initial
version calculates the low transverse momentum contribution to the fully
differential cross-section for two main categories of processes; the inclusive
production of two photons, and inclusive Drell-Yan production. In all cases
resummation up to Next-to-Next-to-Leading Logarithm (NNLL) is included. We aim
to extend the program to several more processes in the near future. The program
is publicly available on Github.
