Calculating dependencies   
!!! All ebuilds that could satisfy "sys-kernel/gentoo-sources" have been masked.
!!! One of the following masked packages is required to complete your request:
- sys-kernel/gentoo-sources-2.6.28-r3 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.28-r2 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.28-r1 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.28 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.27-r9 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.27-r8 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.27-r7 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.27-r6 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.27-r5 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.27-r4 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.27-r3 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.27-r2 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.27-r1 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.27 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.26-r4 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.26-r3 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.26-r2 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.25-r9 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.24-r8 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.20-r10 (masked by: missing keyword)
- sys-kernel/gentoo-sources-2.6.16-r13 (masked by: missing keyword)

For more information, see MASKED PACKAGES section in the emerge man page or 
refer to the Gentoo Handbook.