todo

# Make parameters definition check that group names are not reused also as strategy names
# Replace namespace with something simpler
# - Evaluate everything into environments
# - Check that anything referencing state/model time is a vector of right length
# Consider memoization
# Create a function to calculate expected values from trees
# Add variables at tree nodes
# Automatic collapsing of state-time is a no-go, need to properly propagate references to state_time!!
#   Need to figure out how to do this w/ decision trees!!!
#   Can do this by parsing calls to p(). Safe to do this because you can't pipe
#     into first argument of p() because it uses NSE.
# Try checking state-time dependency inside loop of trans prob evaluation.
# Pull variables from excel workbooks based on named ranges
# Figure out how to handle dependencies of trees (state time!)
# Set attributes on functions to create runtime dependencies?
# Create state groups and allow states within a group to share state_time
# Find and update ###TECHDEBT###

http://jiarubai.net/slides/mdt.pdf