  We present evidence that a magnetic flux rope was formed before a coronal
mass ejection (CME) and its associated long-duration flare during a pair of
preceding confined eruptions and associated impulsive flares in a compound
event in NOAA Active Region 12371. Extreme-ultraviolet images and the
extrapolated nonlinear force-free field show that the first two, impulsive
flares, SOL2015-06-21T01:42, result from the confined eruption of highly
sheared low-lying flux, presumably a seed flux rope. The eruption spawns a
vertical current sheet, where magnetic reconnection creates flare ribbons and
loops, a nonthermal microwave source, and a sigmoidal hot channel which can
only be interpreted as a magnetic flux rope. Until the subsequent long-duration
flare, SOL2015-06-21T02:36, the sigmoid's elbows expand, while its center
remains stationary, suggesting non-equilibrium but not yet instability. The
"flare reconnection" during the confined eruptions acts like "tether-cutting
reconnection" whose flux feeding of the rope leads to instability. The
subsequent full eruption is seen as an accelerated rise of the entire hot
channel, seamlessly evolving into the fast halo CME. Both the confined and
ejective eruptions are consistent with the onset of the torus instability in
the dipped decay index profile which results from the region's two-scale
magnetic structure. We suggest that the formation or enhancement of a
non-equilibrium but stable flux rope by confined eruptions is a generic process
occurring prior to many CMEs.
