Complex sentence: Atlas Autocode included a complex data type which would support complex numbers (for example, the square root of -1), a feature which was dropped when the language later morphed into the Edinburgh IMP programming language.
Split into simpler sentences: Atlas Autocode included a complex data type which would support complex numbers (for example, the square root of -1). This'complex' feature was dropped when Atlas Autocode later morphed into the Edinburgh IMP programming language.