Trial Handler: Making trial order conditional

I am designing a memory task in Psychopy and I have 4 different types of feature search contained (16 total trials with other conditions). Currently I am using trialHandler but it is very important that I ensure that two of the same trial types can not directly follow eachother. So if I have trial types a, b, c and d, I need to make sure I never get b, a, c, c, a in the randomisation of my order. Is there anyone way to manipulate how trialHandler selects it trial order? Thank you