protected UseropDecl userop(DataType returnType, String name, List<DataType> parameterTypes) {
		return new DefaultUseropDecl(this, returnType, name, parameterTypes);
	}
Declare an external userop

@param returnType the userop's "return type"
@param name the name of the userop as it would appear in Sleigh code
@param parameterTypes the types of its parameters, in order
@return the declaration, suitable for generating invocations
Ghidra/Features/SystemEmulation/src/main/java/ghidra/pcode/struct/StructuredSleigh.java