  We describe an explicit in time, finite-difference code designed to simulate
black holes by using the excision method. The code is based upon the harmonic
formulation of the Einstein equations and incorporates several features
regarding the well-posedness and numerical stability of the initial-boundary
problem for the quasilinear wave equation. After a discussion of the equations
solved and of the techniques employed, we present a series of testbeds carried
out to validate the code. Such tests range from the evolution of isolated black
holes to the head-on collision of two black holes and then to a binary black
hole inspiral and merger. Besides assessing the accuracy of the code, the
inspiral and merger test has revealed that individual apparent horizons can
touch and even intersect. This novel feature in the dynamics of the marginally
trapped surfaces is unexpected but consistent with theorems on the properties
of apparent horizons.
