<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def cat_trials(x3d):
 """Concatenate trials along time axis. Parameters x3d : array, shape (t, m, n) Segmented input data with t trials, m signals, and n samples. Returns ------- x2d : array, shape (m, t * n) Trials are concatenated along the second axis. See also -------- cut_segments : Cut segments from continuous data. Examples -------- (4, 900) """

    x3d = atleast_3d(x3d)
    t = x3d.shape[0]
    return np.concatenate(np.split(x3d, t, 0), axis=2).squeeze(0)