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(a) Nonretroactive Application—Nothing contained in this chapter shall be construed to require the removal, lowering, or other change or alteration of any structure or tree not conforming to this chapter as of its effective date, or otherwise interfere with the continuance of a nonconforming use, nor to require any change in the construction, alteration, or intended use of any structure, the construction or alteration of which was commenced prior to the effective date of this chapter, and which is diligently continued to completion.\n(b) Marking and Lighting—Notwithstanding subsection (a), the owner of any existing nonconforming structure or tree is hereby required to permit the installation, operation, and maintenance thereon of such markers and lights as shall be deemed necessary by the Virgin Islands Port Authority to indicate to the operators of the aircraft in the vicinity of the airport the presence of such airport obstruction. Such markers and lights shall be installed, operated, and maintained at the expense of the Virgin Islands Port Authority.